Re-enable app handle animations

Enables two app handle animations:
1) visibility: fade in/out of the whole caption view using the
   pre-existing alpha object animator, plus some additional handling for
   repeated bind calls (to just continue the animation without changing
   duration) and reversing
2) color: interpolate the handle view between the start/end colors, plus
   additional handling for repeated bind calls (to just continue) and
   mid-animation target color updates.

Also changes the color of the handle used when the task's bgColor is
transparent. Previously it would always evaluate to light which is
changed to using the system theme instead for less flickering during
cold-starts when the bgColor tends to be transparent.

Flag: com.android.window.flags.reenable_app_handle_animations
Fix: 412647178
Test: atest AppHandleViewHolderTest
Change-Id: Id363f31b3dfe1c5d16921f5bd81eac21334213f9