Move IMA SDK callbacks into inner class

The release() method was added in the recent IMA API changes for
preloading and now 'collides' with the ExoPlayer AdsLoader release
method. This led to all ads completing being treated as a call to
completely release the ads loader, which meant that the ad playback
state was not updated on resuming after all ads had completed, which
in turn led to playback getting stuck buffering on returning from the
background after all ads played.

Move the IMA callbacks into an inner class to avoid this.

Issue: #7508
PiperOrigin-RevId: 316834561

RELEASENOTES.md

@@ -215,6 +215,9 @@
     *   Add option to skip ads before the start position.
     *   Catch unexpected errors in `stopAd` to avoid a crash
         ([#7492](https://github.com/google/ExoPlayer/issues/7492)).
+    *   Fix a bug that caused playback to be stuck buffering on resuming from
+        the background after all ads had played to the end
+        ([#7508](https://github.com/google/ExoPlayer/issues/7508)).
 *   Demo app: Retain previous position in list of samples.
 *   Add Guava dependency.