[Decor] Support ScreenDecorCommand in ScreenDecorations

This CL ties together the debug support added earlier to the
ScreenDecorCommand that now exists. It also makes the following changes:

1. Allow for nullable top/bottom rounded drawables in the ScreenDecorHwcLayer
2. Make ScreenDecorHwcLayer support dynamic color

Test: manual
Test: existing screendecor and command line tests
Bug: 285941724
Change-Id: I945e5086735b78718fbe388e5567c5f312834ae2