Tweak shrinking guide and update release notes.

The shrinking didn't mention that users of the existing
ProgressiveMediaSource need to pass in ExtractorsFactory.EMPTY to the
SimpleExoPlayer.Builder as well.

Also updated the release notes to mention the changed shrinking
guidance.

Issue: #7937
PiperOrigin-RevId: 333060452

RELEASENOTES.md

@@ -77,6 +77,10 @@ To learn more about what's new in 2.12, read the corresponding
             playable `MediaSource` instances. A `DefaultMediaSourceFactory` is
             used by default. `Builder.setMediaSourceFactory` allows setting a
             custom factory.
+        *   Update [APK shrinking guide](https://exoplayer.dev/shrinking.html)
+            to explain how shrinking works with the new `MediaItem` and
+            `DefaultMediaSourceFactory` implementations
+            ([#7937](https://github.com/google/ExoPlayer/issues/7937)).
         *   Add additional options to `Builder` that were previously only
             accessible via setters.
         *   Add opt-in to verify correct thread usage with