ScreenDecorHwcLayer: better debugging for the transparent region

When DEBUG_COLOR is set, the ScreenDecorHwcLayer becomes a full color
buffer (instead of A8) and draws green for the cutouts and transparent
green for the transparent region. But the old method calculated the
transparent region during every draw call, which doesn't match how it
happens when we actually set the transparent region. Instead, do the
following:

Always call requestTransparentRegion in onAttachedToWindow. This means
that gatherTransparentRegion will be called to calculate the transparent
region. Do *not* call calculateTransparentRect in onDraw, which would
update the rect. The final result is that onDraw will draw using the
rect computed for passing to native.

When DEBUG_COLOR is set, still compute the rect in
gatherTransparentRegion, but set the region to empty. The method is
supposed to remove parts of the Region that are not transparent, and
with the debug drawing, it essentially draws everywhere.

Only call setDisplayDecoration when !DEBUG_COLOR. With DEBUG_COLOR set,
we cannot take advantage of DISPLAY_DECORATION layers anyway.

Bug: 225212638
Test: debug.screenshot_rounded_corners
Change-Id: Ic91d3b22e5c5d768cc1c6f1f8baefe1546ac2006