Add support for building Filterscript files.

This change piggybacks Filterscript rules onto the existing Renderscript
rules. There is no real difference other than the file extension (which
llvm-rs-cc uses to apply a more stringent set of warnings/errors).

Bug: 7166741
Change-Id: I27175ff1c16c27129cafa92b4e8caabf2e5f4150