Makes raw back event sends to the secondary container

Similar to commit 7f21272f, this CL is targeting on raw back key
events that adjusts the focused window to the window that has
higher z-ordering if possible.

In addition, the focus is moved to the top-most embedded activity
in order to accommodate the overlay TF case.

Bug: 240575809
Bug: 322939617
Test: verified on Settings
Test: atest TaskFragmentTest
Change-Id: Iac38655edcb6283c01a6dd98ca0361935f6f46d0