fix: Cache errors when rapidly switching sources

[daytime-em]

Root cause: calling SQLiteDatabase.close() (via use) is not thread-safe, even though it appears to be. This is a problem because media3's loader is multithreaded. The SQLiteDatabase and SQLiteCursor all have guards around their init and closing logic, but the underlying SQLiteConnectionPool isn't guarded adequately in this case. Since a single CacheDatastore can be used by multiple loader threads (as-designed in media3), we can't rely on SQLiteDatabase.close().

Because of all of that, rapidly switching media sources many times (like when fling-scrolling a feed) would occasionally result in playback errors

Solution: Explicitly keep an open SQLiteDatabase object around for as long as a Player needs a CacheDatastore. This is fine to do, and was the general intent of my last attempt at fixing this. Before, I was using acquireReference to hopefully keep the SQLiteDatabase's underlying connection open even if close() was called. This wasn't effective thanks to media3's multithreaded loading behavior. Now, we just keep the DB object (and therefore connection) open until we are closed (when our MuxPlayer is released)

[cjpillsbury]

LGTM!