[Activity Indicator] Don't call UIAppearance in +initialize

[ianegordon]

Closes #2781

[codecov-io]

Codecov Report

Merging #2810 into develop will decrease coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2810      +/-   ##
===========================================
- Coverage    48.46%   48.45%   -0.01%     
===========================================
  Files          165      165              
  Lines        24864    24860       -4     
  Branches      1084     1084              
===========================================
- Hits         12050    12046       -4     
  Misses       12746    12746              
  Partials        68       68

Impacted Files	Coverage Δ
...nents/ActivityIndicator/src/MDCActivityIndicator.m	14.4% <ø> (-0.57%)	⬇️

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