The new callback allows an app to read data from the content
timeline to populate the `AdPlaybackState`. To avoid a deadlock
between the `AdMediaSource` waiting for the `AdPlaybackState` and
the app waiting for the content `Timeline`, a boolean flag
`useLazyContentSourcePreparation` is introduced to tell the
`AdsMediaSource` to prepare the content source immediately
to make the `Timeline` available.
A unit test verifies that in none of the cases (lazy preparation or
immediate preparation) a `Timeline` without ad data is leaked to the
caller. This ensures that in the case of a preroll, the player won't
initially and accidentally read content media data before starting to
load the preroll ad. While the content source is prepared early, no
content media period must be created before the preroll starts.
PiperOrigin-RevId: 696885392
There's no need to manually construct a 'default'
DefaultDataSource.Factory instance, we can just pass the `Context` to
`DefaultMediaSourceFactory` and let it construct the
`DefaultDataSource.Factory` internally.
PiperOrigin-RevId: 451155747
