[AppBar] Fix bug where initialization of nav controller with root view controller would not inject an App Bar.

[jverkoey]

Verified that the added test fails prior to this change and succeeds after.

Closes https://github.com/material-components/material-components-ios/issues/4688

[codecov-io]

Codecov Report

Merging #4691 into develop will decrease coverage by 0.18%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #4691      +/-   ##
===========================================
- Coverage    53.55%   53.36%   -0.19%     
===========================================
  Files          323      323              
  Lines        27921    27899      -22     
===========================================
- Hits         14952    14888      -64     
- Misses       12969    13011      +42

Impacted Files	Coverage Δ
...ponents/AppBar/src/MDCAppBarNavigationController.m	87.25% <100%> (+5.81%)	⬆️
...exibleHeader/src/MDCFlexibleHeaderViewController.m	63.15% <0%> (-9.15%)	⬇️
...ponents/ButtonBar/src/private/MDCButtonBarButton.m	93.33% <0%> (-6.67%)	⬇️
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%)	⬇️
components/Tabs/src/private/MDCItemBar.m	42.75% <0%> (-0.52%)	⬇️
components/Typography/src/MDCTypography.m	91.07% <0%> (-0.47%)	⬇️
components/TextFields/src/MDCMultilineTextField.m	76.62% <0%> (-0.44%)	⬇️
components/Chips/src/MDCChipField.m	16.62% <0%> (-0.27%)	⬇️
... and 7 more

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