Skip snapshot task when finishing activity for shell transition

With shell transition, the snapshot from
ActivityRecord#finishIfPossible
is always replaced by
Transition#onTransactionReady -> TaskSnapshotController#recordSnapshot

And because currently SnapshotController#onTransitionReady is a
dead code so the task added by addSkipClosingAppSnapshotTasks
won't be cleared.

So the duplicated snapshot can be skipped that eliminates the
only one caller that will add task to mSkipClosingAppSnapshotTasks.
Which also makes finishActivity more efficient.

Also change mSkipClosingAppSnapshotTasks to store task id. So it
won't keep heavy references.

Bug: 283177730
Bug: 273198446
Test: TaskSnapshotControllerTest
Change-Id: I3d03c34d506d0f3b1c0e070b8702a6a66a4de3c1