[Chips] Fix accessoryPadding encoding key

[romoore]

Encoding issues a warning because the titlePadding value is overwritten with
the accessoryPadding value during archival.

[codecov-io]

Codecov Report

Merging #2773 into develop will increase coverage by 1.3%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           develop   #2773     +/-   ##
=========================================
+ Coverage    46.89%   48.2%   +1.3%     
=========================================
  Files          165     165             
  Lines        24651   24651             
  Branches      1060    1077     +17     
=========================================
+ Hits         11561   11882    +321     
+ Misses       13027   12701    -326     
- Partials        63      68      +5

Impacted Files	Coverage Δ
components/Chips/src/MDCChipView.m	49.6% <100%> (+27.55%)	⬆️
components/private/Math/src/MDCMath.h	59.13% <0%> (+17.39%)	⬆️
...s/private/ShapeLibrary/src/MDCPillShapeGenerator.m	52.17% <0%> (+17.39%)	⬆️
...mponents/private/Shapes/src/MDCShapedShadowLayer.m	63.85% <0%> (+24.09%)	⬆️
components/private/Shapes/src/MDCEdgeTreatment.m	50% <0%> (+25%)	⬆️
...ivate/ShapeLibrary/src/MDCRoundedCornerTreatment.m	66.66% <0%> (+28.57%)	⬆️
components/private/Shapes/src/MDCPathGenerator.m	38.82% <0%> (+38.82%)	⬆️
...ts/private/Shapes/src/MDCRectangleShapeGenerator.m	67.28% <0%> (+60.28%)	⬆️

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