Include all not occluded leaf tasks while wrapping animation target

There might be other leaf tasks beside the top leaf task are not
occluded. Like the case when split screen is activated, both top leaf
tasks in each side are not occluded. This makes sure to include them
into remote animation target.

Fix: 227307495
Test: pass existing tests
Test: enter split, quick switch back and forth won't crash

Change-Id: Ic8af56b1a9e029dc4f1c2c57ab59071aab2924b4