Add playlist and seek operations to SimpleBasePlayer

These are the remaining setter operations. They all share the same
logic that handles playlist and/or position changes. The logic to
create the placeholder state is mostly copied from ExoPlayerImpl's
maskTimelineAndPosition and getPeriodPositonUsAfterTimelineChanged.

PiperOrigin-RevId: 496364712
(cherry picked from commit 5fa11564)

RELEASENOTES.md

@@ -26,6 +26,8 @@
     *   SubRip: Add support for UTF-16 files if they start with a byte order
         mark.
 *   Session:
+    *   Add abstract `SimpleBasePlayer` to help implement the `Player` interface
+        for custom players.
     *   Add helper method to convert platform session token to Media3
         `SessionToken` ([#171](https://github.com/androidx/media/issues/171)).
 *   Metadata: