SDK / exoplayer

- Increase the default cache file size to 5MB
- Recommended a minimum cache file size of 2MB to discourage
  applications from specifying values small enough such that
  unreasonably large numbers of cache files are generated
- Allow maxCacheFileSize=C.LENGTH_UNSET, equivalent to setting it
  to MAX_VALUE. This is just for API consistency with other APIs
  we have that accept LENGTH_UNSET

Issue: #4253
PiperOrigin-RevId: 227524233