libstagefright: Make timed text for audio only files work on start

The basic problem is for an audio-only track adding the subtitle track through
the Android api does not kick off the text driver when you call start

Change-Id: I86d206551fa783c225c1867122148c68ef41404a
Merge remote-tracking branch 'upstream/kk4.4-caf' into kk4.4-caf