Use opaque enter animation for freezing without rotation change

Currently the alpha of root surface applies to child surfaces
individually instead of the composited result. To polish the
see through effect, keep the enter animation with alpha 1.

Bug: 185403130
Test: Toggle dark theme from quick setting.
      The app behind notification shade won't be slightly visible.
Change-Id: I88d93a5e365a50b3965f634c20741450e8127fba