SDK / exoplayer

We need to support serialization to/from an SQLite table. The
model of passing something around for each class to write into
doesn't work well for SQL, and it would be messy to have two
different structural designs for serialization. This change
centralizes the logic in CachedContentIndex, where a centralized
SQL based version can more easily sit alongside it.

PiperOrigin-RevId: 230692291