Notify ITaskStackListener#onTaskMovedToFront when root task moves

The callback was used to be called only when the leaf task
moved to front. However, it should also be called if the top
leaf task is not moved, but the parent root task is moved to
front.

Bug: 338612247
Test: TaskDisplayAreaTests
Flag: EXEMPT bugfix
Change-Id: I09463b0cefa013238e49e06c7d2b9f3a5d74189c