The MP4 data in JPEG motion photos can contain multiple `video/hevc` tracks, but only the first is at a playable frame rate while the others are low-fps, high-res tracks designed for specific use-cases (not direct video playback).
ExoPlayer currently selects the unplayable track by default, because it
has a higher resolution. This change introduces a flag to
`Mp4Extractor` that results in the first video track being marked as
`ROLE_FLAG_MAIN`, and all subsequent video tracks `ROLE_FLAG_ALTERNATE`
- this then results in the playable lower-res track being selected by
default.
PiperOrigin-RevId: 589832072
5266c71b3a