fix(list): Accept array of index for selectedIndex API

[abhiomkar]

Before this implementation selectedIndex component API (that proxies foundation's setSelectedIndex) use to accept only single number index. This change adds support for multiple indexes that sets the state of checkboxes.

New behaviour for checkbox based list

[ ] option 1
[ ] option 2
[ ] option 3
[ ] option 4

selectedIndex = [1, 3];

[ ] option 1
[x] option 2
[ ] option 3
[x] option 4

selectedIndex = [2, 3];

[ ] option 1
[ ] option 2
[x] option 3
[x] option 4

selectedIndex = [];

[ ] option 1
[ ] option 2
[ ] option 3
[ ] option 4

selectedIndex = 2;

// Throws error

Other changes:

• getter API for selectedIndex that returns the current state of selection for all variants.
• Tabindex is set to 0 for currently focused element and will be reverted to first selected element on focus out of list root.
• new adapter method isFocusInsideList.

BREAKING CHANGE: Introduced new adapter isFocusInsideList for MDC List for improved accessibility.

Fixes #4122
Fixes #3611

[mdc-web-bot]

All 679 screenshot tests passed for commit 7420515 vs. master! 💯🎉

[mdc-web-bot]

All 679 screenshot tests passed for commit e5664d9 vs. master! 💯🎉

[kfranqueiro]

FYI it looks like there are some closure failures to address, and there's a seemingly spurious unit test failure in the last run... Screenshot tests should pass once this is synced with master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@96f663e). Click here to learn what that means.
The diff coverage is 98.47%.

@@            Coverage Diff            @@
##             master    #4124   +/-   ##
=========================================
  Coverage          ?   98.49%           
=========================================
  Files             ?      127           
  Lines             ?     5697           
  Branches          ?      761           
=========================================
  Hits              ?     5611           
  Misses            ?       86           
  Partials          ?        0

Impacted Files	Coverage Δ
packages/mdc-list/constants.js	100% <ø> (ø)
packages/mdc-list/foundation.js	100% <100%> (ø)
packages/mdc-list/index.js	98.49% <87.5%> (ø)

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[mdc-web-bot]

All 758 screenshot tests passed for commit 2ea1d3b vs. master! 💯🎉

[mdc-web-bot]

All 758 screenshot tests passed for commit 38a34de vs. master! 💯🎉