Clean up makefile to fix build.

mediafilter was defining LOCAL_NDK_STL_VARIANT, but never setting
LOCAL_SDK_VERSION, so it was a no-op. Since the project was still
manually setting the stlport include path, it was getting the stlport
headers with precedence over libc++, which breaks the world.

Once that is fixed, the project has never linked libRScpp, which is
required (I'm not sure how this part has ever worked).

Change-Id: Iac70c86bc150eea08329812c0d5744434318d28e