Desktop Windowing: add sysprop flags for shadows and rounded corners

We add three flags:
1. Enable/disable rounded corners (boolean flag)
2. Enable/disable window shadows (boolean flag)
3. Enable/disable window shadows for the focused window (boolean flag) -
this flag only applies if flag 2 is disabled.

This CL does NOT update default bevahiours, i.e. in desktop mode, window
shadows and rounded corners are still shown by default.

adb commands: https://paste.googleplex.com/6310119139180544

Screenshot with rounded corners:
https://screenshot.googleplex.com/BHw6hDPnSa8a8yN.png
Screenshot without rounded corners:
https://screenshot.googleplex.com/98VVVd3Jk8y8dnF.png
Note: the screenshots don't seem to capture window shadows.

Bug: 316867234, 316865378
Test: unit tests for default behavior, ran adb commands to check sysprop
flags behavior

Change-Id: Iaafb50ebf82270c8e38bd957176d83ce6182a6dd