Build system support for RenderScript

With this change, you can add your .rs files to LOCAL_SRC_FILES.
The .rs files will be compiled by slang and the output .java files
will be compiled into the jar, while the generated .bc files will
be put into the final apk as raw resources.

Change-Id: Icde3d6139951d6d039b4b37376e72e1fc5c8b0d4