[Snackbar] Create explicit singleton

[romoore]

Creating an explicit singleton for the MDCSnackbarManager so that clients can
use their own instances (perhaps one per screen in the future) or configure
instances for testing.

Unblocks internal issue b/77900596

This is a roll-forward of #4556
Closes #4686

[material-automation]

API diff detected the following changes

Snackbar

MDCSnackbarManager

new method: -setPresentationHostView: in MDCSnackbarManager

new method: -suspendAllMessages in MDCSnackbarManager

new method: -suspendMessagesWithCategory: in MDCSnackbarManager

new property: messageTextColor in MDCSnackbarManager

new property: snackbarMessageViewShadowColor in MDCSnackbarManager

new method: -setBottomOffset: in MDCSnackbarManager

new method: -hasMessagesShowingOrQueued in MDCSnackbarManager

new method: -dismissAndCallCompletionBlocksWithCategory: in MDCSnackbarManager

new property: buttonFont in MDCSnackbarManager

new property: mdc_adjustsFontForContentSizeCategory in MDCSnackbarManager

new method: -buttonTitleColorForState: in MDCSnackbarManager

new property: alignment in MDCSnackbarManager

new property: delegate in MDCSnackbarManager

new property: defaultManager in MDCSnackbarManager

new method: -showMessage: in MDCSnackbarManager

new property: messageFont in MDCSnackbarManager

new method: -setButtonTitleColor:forState: in MDCSnackbarManager

new method: -resumeMessagesWithToken: in MDCSnackbarManager

new property: snackbarMessageViewBackgroundColor in MDCSnackbarManager

new property: shouldApplyStyleChangesToVisibleSnackbars in MDCSnackbarManager

MDCSnackbarManager(LegacyAPI)

new category: MDCSnackbarManager(LegacyAPI)

modified class method: +buttonTitleColorForState: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +setBottomOffset: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: mdc_adjustsFontForContentSizeCategory in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +hasMessagesShowingOrQueued in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +suspendMessagesWithCategory: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: snackbarMessageViewBackgroundColor in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: shouldApplyStyleChangesToVisibleSnackbars in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: delegate in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +suspendAllMessages in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: buttonFont in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +setPresentationHostView: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +dismissAndCallCompletionBlocksWithCategory: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +setButtonTitleColor:forState: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: alignment in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: snackbarMessageViewShadowColor in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +showMessage: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: messageTextColor in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified class method: +resumeMessagesWithToken: in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

modified property: messageFont in MDCSnackbarManager(LegacyAPI)

Type of change:	parent.usr
From:	c:objc(cs)MDCSnackbarManager
To:	c:objc(cy)MDCSnackbarManager@LegacyAPI

[codecov-io]

Codecov Report

Merging #4742 into develop will increase coverage by 0.01%.
The diff coverage is 90.13%.

@@            Coverage Diff             @@
##           develop   #4742      +/-   ##
==========================================
+ Coverage    53.48%   53.5%   +0.01%     
==========================================
  Files          323     323              
  Lines        27977   28025      +48     
==========================================
+ Hits         14964   14994      +30     
- Misses       13013   13031      +18

Impacted Files	Coverage Δ
...ckbar/src/private/MDCSnackbarMessageViewInternal.h	100% <ø> (ø)	⬆️
components/Snackbar/src/MDCSnackbarMessageView.m	51.15% <100%> (+0.43%)	⬆️
components/Snackbar/src/MDCSnackbarManager.m	86.89% <88.97%> (+11.95%)	⬆️
...exibleHeader/src/MDCFlexibleHeaderViewController.m	63.63% <0%> (-9.1%)	⬇️
...ponents/ButtonBar/src/private/MDCButtonBarButton.m	93.33% <0%> (-6.67%)	⬇️
...vigation/src/private/MDCBottomNavigationItemView.m	66.66% <0%> (-4.97%)	⬇️
...ents/BottomNavigation/src/MDCBottomNavigationBar.m	62.59% <0%> (-4.16%)	⬇️
components/TextFields/src/MDCTextField.m	74.11% <0%> (-3.32%)	⬇️
components/NavigationBar/src/MDCNavigationBar.h	70% <0%> (-2.73%)	⬇️
components/ButtonBar/src/MDCButtonBar.m	64.49% <0%> (-1.82%)	⬇️
... and 15 more

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