move mpd_options to its own file

Also always generates sidx box for segments for now.

Change-Id: I61f71ebd281195b737902b57557a345c8e0b0c66
Use relative paths for files referenced in the MPD (2nd round).

Change-Id: I929b777dc3353fb868489b42e11578d2aa075510
MPD builder now generating relative paths if possible.
MPD builder will now adjust paths if they can be relative to the MPD file.

Change-Id: I319ae6ba63783d9b08fe80a875a5ddfb1e9ca322