Better IME transition while switching app with recents (2/N)

In TaskSnapshotController#handleClosingApps will capture task snapshot
while applying closing animation or during screen turning-off.

this will overwrite the captured task from RecentsAnimationController,
and makes while quick switching tasks, sometimes will not see the IME
surface on the task snapshot.

Add a check in places which triggers task snapshot to ignore addidnal
snapshot request while RecentsAnimation is active.

Also refined Task#isTaskAnimating as isAnimatingByRecents to simplify
the logic and add the test for its behavior.

Bug: 166736352
Test: manual by:
      1) Tapping EditText in app and make keyboard shown
      2) Quick switch back and force between apps by swiping navbar
      3) Verify if the task snapshot with IME persists shown during
         switching apps
Test: atest RecentsAnimationControllerTest#testIsAnimatingByRecents

Change-Id: Icc7abd3338f472e263042f46502615f0446b46eb