Fix window visibility during layout

Avoid the conditional requestTransparentRegion() and make it more
precise in onAttachedToWindow().

Testing: 20 attempts to launch in TreasureHunt, never results in black
screen as compared to 2-3 attempts to get into black screen before.
Bug: 62235292

Suggested-By:  <racarr@google.com>
Change-Id: If04c97670f24c10bf9e4ce95b9d33fb750053323