Per-display focused stacks in AM

This switches to tracking focused stacks for each display instead of
a single focused stack in the system.

Now ActivityDisplay#getFocusedStack() will return the corresponding
instance, if present on display. Previously it was tracked in
ActivityStackSupervisor with a field. Now the getter searches for a
focusable stack starting from top. Usually it will be the first one
- only PiP stack can be on on top and not focused.

ActivityStackSupervisor#getFocusedStack() will search across displays
for the topmost focusable stack  based on the order provided by WM.

The assumption that there always exists a focused stack is still true
- ActivityStackSupervisor#getFocusedStack() should never return null.
There always exists a focusable home stack on the default display.

Bug: 111541062
Test: atest com.android.server.am
Change-Id: I92922e4298ebbe1c68791bf59406e8f387bcd564