Fix bug with flickering actions bar

This CL fixes a bug caused by ag/27148514. The bug occurred because:

1) The new way we use FloatProperty to set the alpha did not have a working `get()` function, so it just returned a junk value, set to `-1f` (I didn't think `get()` would ever be called meaningfully for this property).

2) However, during certain UI interactions, `RecentsView#updateActionsViewFocusedScroll()` is called many times in a loop, which repeatedly tries to re-start the fade-in animation for actions bar. Pre ag/27148514, even though the fade-in animation was called repeatedly, it would start and then immediately stop running because it realized that it was already set to the desired alpha. However now that we return a junk value for `get()`, it didn't know to skip and just ran the full fade-in animation again and again.

Fixed by refactoring FLOAT_SETTER (now ALPHA_PROPERTY) and creating a new data class, OverviewActionsAlphaProperty, that handles some of the intricacies of this increasingly complicated alpha system.

Fixes: 339545510
Test: Swipe to Overview, move tiles around, actions bar does not flicker.
Flag: ACONFIG com.android.wm.shell.enable_app_pairs NEXTFOOD
Change-Id: I1093363780db5e0780deda2ff14b4f7361d63940
Merged-In: I1093363780db5e0780deda2ff14b4f7361d63940