fix default subtitle not turning on when not transcoding, and burnt in subs not turning off

[mueslimak3r]

Changes

• made the play() parameter transcodedSubtitle a nullable Integer so that we can specifiy null to defer to the server default, -1 to disable subs, and >=0 to select burnt in/transcoded subs
• reworked the logic for determining if a newly started stream is using burnt in subs, and set relevant flags locally so no additional restarts are needed
• always set mDefaultSubIndex to the server default so that users will always get subs enabled by default if they have such a preference set.

Issues

• default subtitles for an item wouldn't turn on by default when direct playing
• turning on burnt in subtitles would start the stream twice. After the first startup, switchSubtitleStream would incorrectly assume that the stream didn't already have the burnt in subs enabled, and restart the stream - this time the necessary flags were set to not require more restarts.
• burnt in subtitles couldn't be disabled during playback
• potentially fixes subtitle settings for a previous episode persisting into the next one, and showing the wrong subs