SurfaceFlinger: Call libQcomUI's clearRegion instead of glClear

Call libQcomUI's implementation of clearRegion instead of glClear. This
implementation is specifically designed to optimize the drawWormhole for non GPU
composition cases, in order to improve performace. If the qcom version
of this API returns an error, we fallback to SurfaceFlinger's implementation
of drawWormhole. The clearRegion API is only called if there are no layers
being drawn by the GPU.

Change-Id: I1a50cf5ba2a88ffcadf13ff78df16f5a30dfff3a