Renderscript: Assume highest API version if not specified

RS_TARGET_API now defines development kit platform version.
If not specified assume highest currently available version,
which is also the only one supporting Renderscript in
Android NDK. In the future, this define will be removed
completely.

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extra : rebase_source : f0b8e504b0420647994312a9a4fa117c70a3256a