Support enabling next/previous actions in PlayerNotificationManager

The ref'd issue was marked as a doucmentation candidate, but I think
the confusion likely arises from the lack of "next" and "previous" in
the method names. Our other UI components also support enabling each
button individually, so this also brings notifications in line with
those.

Issue: #6491
#exofixit
PiperOrigin-RevId: 344058969

RELEASENOTES.md

@@ -51,6 +51,8 @@
     *   Add `bar_gravity` attribute into `DefaultTimeBar`.
     *   Add `bar_gravity` attribute into `DefaultTimeBar`.
     *   Increase seekbar's touch target height in `StyledPlayerControlView`.
     *   Increase seekbar's touch target height in `StyledPlayerControlView`.
     *   Update Styled Player settings dialogs to respect RTL.
     *   Update Styled Player settings dialogs to respect RTL.
+    *   Support enabling the previous and next actions individually in
+        `PlayerNotificationManager`.
 *   Audio:
 *   Audio:
     *   Retry playback after some types of `AudioTrack` error.
     *   Retry playback after some types of `AudioTrack` error.
     *   Work around `AudioManager` crashes when calling `getStreamVolume`
     *   Work around `AudioManager` crashes when calling `getStreamVolume`