[RenderScript] Make support lib harder to leak context.

  - For compat lib, the created context will be cached for future reuse
    within the process now.
  - For native renderscript, the caching mechanism will be enbled for
    API 23 and above.
  - A createMultiContext() is added to support applications need more
    than one RenderScript Context to be used at the same time.

Change-Id: Iaddc0995bfcb1aec0244bdbeed834a33edf8fc7c
1 file changed