[PiP2] Add PipAlphaAnimatorTest

Added the PipAlphaAnimatorTest for PipAlphaAnimator class.
PipAlphaAnimator class is optimized for testing as well in this change

- Allow supplying SurfaceControlTransactionFactory for testing purpose
- Use the local animation listener since they are implementation details
- Switch over to AnimatorListenerAdapter to avoid unnecessary,
  un-testable functions such as onAnimationRepeat

Flag: com.android.wm.shell.enable_pip2
Bug: 376133026
Test: atest WMShellUnitTests:PipAlphaAnimatorTest
Change-Id: I1339cf352c810ea3f66402babc04b6caffed5360