Traverse all leaf task when processing activities visibility

To properly show top activities in multiple non-occluded child tasks,
traverses all leaf tasks when processing activities visibility instead
of processing it only in stack level.

Bug: 171198324
Test: atest TaskTests
Test: manual check split & bubbles behavior
Change-Id: Ic1554903f6623682c183c91dcbb1d947ed8bee55
2 files changed