Support alpha for SurfaceView

Basically, this removes an excuse for apps to use TextureView for video
playback or one-off HDR images prior to HDR UI being ready everywhere.

The idea is that for Z-above, applying alpha is easy, because the
surface just needs to be have alpha modulated. Z-below is a little bit
more creative - the alpha is applied to the hole punch and drawn using
DST_OUT blending semantics. This allows for views underneath the
SurfaceView to blend with surface, while being occluded by views on top
of the SurfaceView.

There may need to be some complex view hierarchies that would be useful
to test, but simple view layouts seem to work.

Note that this is guarded with a target SDK check, to defend against
applications that are propagated alpha to child views while expecting
opaque SurfaceViews.

Bug: 241474646
Test: HWAccelerationTest doesn't look broken
Change-Id: Ibc14b18f1ce6f25250318db50275c6b8c972bade