Simplify check-behind-fullscreen-activity

Since the general traversal methods getOccludingActivityAbove
and forAllActivities are available, the complex helper can
be replaced with the combination of traversal methods.

Bug: 160776093
Test: atest ActivityStackTests#testIterateOccludedActivity
Change-Id: Ifd821a12a0148d7e51b145a507ef8b54ccbdc531