Fix app requested animation for ActivityEmbedding remote animation

1. Clear mNextAppTransitionBackgroundColor after the current app
   transition has been played or aborted.
2. Do not reset the app requested animation when the app transition is
   going to be animated by ActivityEmbedding remote handler, so that we
   can still respect app's requested background color.

Bug: 241043533
Bug: 254090083
Test: atest WmTests:AppTransitionTests
Test: manual verify ActivityEmbedding remote animation with app calling
      Activity#overridePendingTransition
Change-Id: Ib650a7c97ed4ff9341bff51ce9e38c2de3cc1f2d