dvr_api GetExternalSurface returns ANativeWindow

Now that we have a public NDK API to convert an ANativeWindow to a Java
Surface, we no longer need to mess with Jni and libandroid_runtime in
libdvr.

See more information at:
http://ag/#/q/topic:ANativeWindow_toSurface+(status:open+OR+status:merged)

Also, as a side effect, we are now free to convert this library to use
Android.bp (though we probably want to hold off for a while).

Bug: 36266201
Test: Built and ran dvr_buffer_queue-test
Change-Id: Ia0f6a8735c7d508195c30e3c7d040791e13a6ce3