[BottomNavigation] Add NSCoding tests

[romoore]

BottomNavigation needs to fully support NSCoding so that clients can use state
restoration if desired.

Closes #2737

[codecov-io]

Codecov Report

Merging #2740 into develop will increase coverage by 1.51%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2740      +/-   ##
===========================================
+ Coverage    45.38%   46.89%   +1.51%     
===========================================
  Files          165      165              
  Lines        24493    24630     +137     
  Branches      1013     1062      +49     
===========================================
+ Hits         11116    11551     +435     
+ Misses       13316    13016     -300     
- Partials        61       63       +2

Impacted Files	Coverage Δ
...igation/src/private/MDCBottomNavigationItemBadge.m	74.66% <100%> (+74.66%)	⬆️
...ents/BottomNavigation/src/MDCBottomNavigationBar.m	58.99% <100%> (+47.87%)	⬆️
...vigation/src/private/MDCBottomNavigationItemView.m	84.73% <100%> (+84.73%)	⬆️

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