feat(tab-bar): Support manual and automatic activation behavior

[kfranqueiro]

Fixes #3286.

BREAKING CHANGE: Adds focusTabAtIndex and getFocusedTabIndex MDCTabBarAdapter APIs; adds focus MDCTab component API used by MDCTabBar.

[mdc-web-bot]

All 176 screenshot tests passed for commit fcfb30a vs. master! 💯🎉

[mdc-web-bot]

All 176 screenshot tests passed for commit 67016f8 vs. master! 💯🎉

[codecov-io]

Codecov Report

Merging #3303 into master will decrease coverage by 0.16%.
The diff coverage is 82.69%.

@@            Coverage Diff             @@
##           master    #3303      +/-   ##
==========================================
- Coverage   98.19%   98.03%   -0.17%     
==========================================
  Files         120      120              
  Lines        5151     5184      +33     
  Branches      644      649       +5     
==========================================
+ Hits         5058     5082      +24     
- Misses         93      102       +9

Impacted Files	Coverage Δ
packages/mdc-tab-bar/constants.js	100% <ø> (ø)	⬆️
packages/mdc-tab/index.js	96.07% <0%> (-3.93%)	⬇️
packages/mdc-tab-bar/foundation.js	100% <100%> (ø)	⬆️
packages/mdc-tab-bar/index.js	88.88% <30%> (-11.12%)	⬇️

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