android: Only reupload subtitle surfaces if they have changed

This avoids reuploading a full resolution 32 bpp surface for
every frame while the subtitles are displayed.

Ideally this information perhaps should be signalled from the vlc
core in some way, but until then, this avoids excessive slowdowns
with subtitles with the mediacodec direct rendering.

Signed-off-by: Martin Storsjö <martin@martin.st>
contrib/orc: use our own download mirror