Improve RenderEngine's blur performance.

This CL uses new Skia APIs to skip the unnecessary glClear issued when
creating the temporary surfaces. On a Pixel 5 device this showed an
~6% improvement in total GPU time as well as a similar improvement on
the CPU time taken by Skia to isssue the calls to the GPU.

Test: perfetto traces
Bug: 176903027
Change-Id: If04b795ce44107670e9e752b10ab5470393e1e32