[Decor] Add DebugRoundedCornerDelegate

This CL introduces the ability to have debug rounded corners in
ScreenDecorations. It does this by creating a DebugRoundedCornerDelegate
that maintains the debug state for rounded corners, and modifying
ScreenDecorations to read from that delegate when in debug mode AND the
delegate has providers.

This means that debug mode actually has 2 stages:
1. simply turning on debug mode and providing no extra rounded corner
  information will change the device-default corners to show up in
  screenshots and display in color.
2. Secondly, providing a debug corner path spec will switch from using
   the rounded corner delegate to the new debug delegate. This switching
   is to-be-defined in a future CL, but it will involve re-solving for
   the providers and overlays.

Test: manual
Test: ScreenDecorationsTest
Bug: 285941724
Change-Id: Ia0ce4c06117a2877cc930a1c1683912a383a968c