SDK / exoplayer

Common houses DataSource as an interface for reading data,
but most of the concrete implementations are in ExoPlayer.
This means that in practice, if an app wants to use a module
that reads using DataSource (e.g. extractor), they may be
forced to depend on ExoPlayer as well to get a concrete
implementation (e.g. FileDataSource). This change moves the
DataSource implementations into common to resolve this.

PiperOrigin-RevId: 403222081