Don't fill boundary rectangle when drawing shadows.

SurfaceFlinger splits up EffectLayers containing shadows into multiple
parts:
1. A layer only containing the shadow metadata, and
2. Either a background blur or a solid color.

Since the default solid color is an opaque black, this causes shadows to
black out PIP windows.

A follow-up patch should change the default solid color to be a
transparent black instead so that nothing is drawn, or define a
canonical invalid color but we'd have to check callers to make sure
the GLES path doesn't break either.

Bug: 175824511
Bug: 175819287
Test: PIP window
Change-Id: I21336a7885299a127f3dad2a6b88c9dcab0b5980