Update skip silence algorithm

The updated algorithm has two main improvements:
 - The silence padding is not constant but a ratio of the original
   silence (up to a defined max) to more naturally represent the
   original gaps.
 - The silence is not instantly going to zero, but uses a ramp down
   and up for a smooth volume transition and also retains a small
   percentage of the original noise for more natural "silences" that
   still contain some background noise.

#minor-release

Issue: google/ExoPlayer#7423
PiperOrigin-RevId: 602322442
(cherry picked from commit bb53333)

Author: tonihei

RELEASENOTES.md

@@ -38,6 +38,13 @@ This release includes the following changes since the
     *   Extract audio types from TS descriptors and map them to role flags,
         allowing users to make better-informed audio track selections
         ([#973](https://github.com/androidx/media/pull/973)).
+*   Audio:
+    *   Improve silence skipping algorithm with smooth volume ramp, retained
+        minimal silence and more natural silence durations
+        ([#7423](https://github.com/google/ExoPlayer/issues/7423)).
+*   Video:
+*   Text:
+*   Metadata:
 *   Image:
     *   Add support for DASH thumbnails. Grid images are cropped and individual
         thumbnails are provided to `ImageOutput` close to their presentation