Stream constructors are now private and streams can be constructed with new Builder classes per stream class. This change has been made to prevent creating and using several constructors in stream classes.
Some default cases have been also added in these Builder classes, so not everything has to be set, depending of the service and the content.
* merge SubtitlesFormat into MediaFormat
* implement Stream interface on Subtitle class
* misc fixes: dont show a snackbar error on age-restricted videos, lint fix.