Add always on top feature support

Add basic functionalities for always on top feature.

- Add a new flag to WindowConfiguration to represent a task wanting to
be on top.
- Update the logic on changing the z-order of windows to make sure
always on top windows are placed above other windows.

Bug: 69370884
Test: go/wm-smoke
Test: atest DisplayContentTests
Test: Used ArcCompanionLibDemo app to verify that when always-on-top is
      set, the app is above the other Android apps and Chrome windows.

Change-Id: Ie8edeb8ceeed0b9ec154b6031ed6cbe7ecc65b12