The `main` role distinguishes a track from an `alternate`, but unlike
`SELECTION_FLAG_DEFAULT` it doesn't imply the track should be selected
unless user preferences state otherwise. e.g. in the case of a text
track, the player shouldn't enable subtitle rendering just because a
`main` text track is present in the manifest.
The `main`/`alternate` distinction is still available through
`Format.roleFlags` and the `ROLE_FLAG_MAIN` and `ROLE_FLAG_ALTERNATE`
values.
This behaviour was originally [added in 2.2.0](https://github.com/google/ExoPlayer/commit/7f967f305718bc2c9ee679fdd7d014eccef0356b),
however at the time the `C.RoleFlags` IntDef did not exist. The IntDef
was [added in 2.10.0](https://github.com/google/ExoPlayer/commit/a86a9137be5f0ed89de3d68f4c4800a7753cc881).
PiperOrigin-RevId: 418937747
