Fix flicker while resizing split screen

Client app in split screen without focus was considered as occluded
unexpectedly while applying transaction to resize split screen, so it
won't process the new size config until the client app been relaunched
due to configuration changed later and thus introduce a flicker. Refines
ActivityRecord#shouldBeVisible to respect leaf task occluding state.

Fix: 228569855
Test: client app on non-focused side of the split screen won't flicker
      after resized
Change-Id: I19ac501526c6a4662609ef84532dc093acb63f54