This adds the API surface for media button preferences in MediaSession
and MediaController. It closely mimics the existing custom layout
infrastructure (which it will replace eventually).
Compat logic:
- Session:
- When converting to platform custom actions, prefer to use
media button preferences if both are set.
- When connecting to an older Media3 controller, send the
media button preferences as custom layout instead.
- Controller:
- Maintain a single resolved media button preferences field.
- For Media3 controller receiving both values, prefer media
button preferences over custom layouts.
Missing functionality:
- The conversion from/to custom layout and platform custom actions
does not take the slot preferences into account yet.
PiperOrigin-RevId: 686950100