Render window decoration in WM shell

There are still several things missing in this CL:

1. Still need to wire up freeform resizing. This is done in a separate
   CL.
2. Check if a11y works as intended.
3. Wire up fullscreen and window decorations for freeform displays.
4. Immersive mode work.

Bug: 165794880
Bug: 165794636
Test: Caption shows up.
Change-Id: I7c0e1d55d6c9d03332a58d31ade88aef06a84048