Move task to the bottom when force hidden

Move a task to the bottom when the task was hidden to prevent
the top focused activity unable to enter pip (while pausing)
due to a invisible task remained on top.

Move the task to top once being visible again

Bug: 180405672
Test: atest TaskTests PinnedStackTests
Change-Id: I038076c58cd41cc90039c9db5a295da5d5ee2cde