Update DownloadService for Android 12

- If DownloadService is configured to run as a foreground service,
  it will remain started and in the foreground when downloads are
  waiting for requirements to be met, with a suitable "waiting for
  XYZ" message in the notification. This is necessary because new
  foreground service restrictions in Android 12 prevent to service
  from being restarted from the background.
- Cases where requirements are not supported by the Scheduler will
  be handled in the same way, even on earlier versions of Android.
  So will cases where a Scheduler is not provided.
- The Scheduler will still be used on earlier versions of Android
  where possible.

Note: We could technically continue to use the old behavior on
Android 12 in cases where the containing application still has a
targetSdkVersion corresponding to Android 11 or earlier. However,
in practice, there seems to be little value in doing this.
PiperOrigin-RevId: 398720114

RELEASENOTES.md

@@ -14,6 +14,10 @@
     *   Move `Player.addListener(EventListener)` and
     *   Move `Player.addListener(EventListener)` and
         `Player.removeListener(EventListener)` out of `Player` into subclasses.
         `Player.removeListener(EventListener)` out of `Player` into subclasses.
 *   Android 12 compatibility:
 *   Android 12 compatibility:
+    *   Keep `DownloadService` started and in the foreground whilst waiting for
+        requirements to be met on Android 12. This is necessary due to new
+        [foreground service launch restrictions](https://developer.android.com/about/versions/12/foreground-services).
+        `DownloadService.getScheduler` will not be called on Android 12 devices.
     *   Disable platform transcoding when playing content URIs on Android 12.
     *   Disable platform transcoding when playing content URIs on Android 12.
     *   Add `ExoPlayer.setVideoChangeFrameRateStrategy` to allow disabling of
     *   Add `ExoPlayer.setVideoChangeFrameRateStrategy` to allow disabling of
         calls from the player to `Surface.setFrameRate`. This is useful for
         calls from the player to `Surface.setFrameRate`. This is useful for
@@ -23,6 +27,14 @@
     *   `SubtitleView` no longer implements `TextOutput`. `SubtitleView`
     *   `SubtitleView` no longer implements `TextOutput`. `SubtitleView`
         implements `Player.Listener`, so can be registered to a player with
         implements `Player.Listener`, so can be registered to a player with
         `Player.addListener`.
         `Player.addListener`.
+*   Downloads and caching:
+    *   Modify `DownloadService` behavior when `DownloadService.getScheduler`
+        returns `null`, or returns a `Scheduler` that does not support the
+        requirements for downloads to continue. In both cases, `DownloadService`
+        will now remain started and in the foreground whilst waiting for
+        requirements to be met.
+    *   Modify `DownlaodService` behavior when running on Android 12 and above.
+        See the "Android 12 compatibility" section above.
 *   RTSP:
 *   RTSP:
     *   Support RFC4566 SDP attribute field grammar
     *   Support RFC4566 SDP attribute field grammar
         ([#9430](https://github.com/google/ExoPlayer/issues/9430)).
         ([#9430](https://github.com/google/ExoPlayer/issues/9430)).