Handle getting the audio track's position before the first AC-3 buffer.

ac3Bitrate is set only after the first buffer is handled, which meant that
getting the playback position would cause a divide by zero before then.

When playing back AC-3 content, the ac3Bitrate will always be set after the
first buffer is handled, so return a 0 position if it is not set.
Enforce sliding window of available segments for DASH DVB.
Identify AC-3 tracks by codecs="ac-3", not the MIME type.
Fix crash running ExoPlayer demo on JB. My bad!

- We can't refer to UnsupportedSchemeException outside of the
V18 compat inner classes.
- There were also a few missing return; calls.
Clear subtitle when text disabled.