Fix activity leak bug

It is possible for ViewRootImpl.doDie() to be called prior to one of the
other calls which tells the system that the window is going to the
foreground. This race condition can cause an activity leak since the
ViewRootImpl in question is stored in a collection of "requestors" which
determine when to pause/resume animators for background/foreground apps.

The fix is two-fold:
- Only cause an item to be added to the "requestor" list in
  AnimationHandler when doDie() has not yet been called (marked by
  the mReoved" flag)
- Store the requestor objects as WeakReferences, rather than the objects
  themselves. Thus if the requestor list is the only referent to the
  object, it can be collected anyway, removing the leak potential.

Bug: 258616235, 258616235
Test: AnimatorLeakTest passes
Change-Id: I7dfebac90aa43f89433f49e0ed60b3ff57bcc497