Override buffer always sets blend mode and plane alpha

This fixes a caching issue where split screen chrome blanks out the
other side of the split screen, because chrome uses an opaque layer and
is the first layer of the cached set. The opaque bit must not be
propagated to composer, since the cached buffer is allowed to blend with
other layers if there are undamaged regions. Similarly, plane alpha is
already applied when generating the override buffer, so we should not
re-apply a non-zero plane alpha.

Bug: 183480321
Test: libcompositionengine_test
Test: split screen with chrome and recent screen

Change-Id: I22ef304409f15f10435d824e5fab6d0c075d1b9a