Refactors the DefaultMediaNotificationProvider by separating the
selection of actions and building the notification with it.
The custom commands of the custom layout of the session are turned
into notification actions and when received from the notification
converted back to custom session commands that are sent to the
session.
PiperOrigin-RevId: 450404350
Define MediaNotification.Provider so that apps can customize
notification UX. Move MediaNotificationManager's functionality
around notifications on DefaultMediaNotificationProvider
PiperOrigin-RevId: 428024699
Where this introduced an inconsistency (e.g. assigning to something
called `windowIndex`), I generally renamed the transitive closure of
identifiers to maintain consistency (meaning this change is quite
large). The exception is code that interacts with Timeline and Window
directly, where sometimes I kept the 'window' nomenclature.
#minor-release
PiperOrigin-RevId: 407040052
