Use vendor variant of libstagefright_foundation.

Rather than having a lib called libstagefright_foundation and one
called libstagefright_foundation_vendor, we now just have one library
and the build system keeps track of the vendor APIs that are exposed.

Test: (sanity) boot device, watch (part of) DRM protected movie.
Change-Id: I7495be3e8f546cac1b4909108313cf61fc85b1d8