Fix available end time calculation for multi-period DASH live streams

The available end time was accidentally substracted by the start time
of the last period.

To avoid similar time reference confusion in the future, also renaming
many variables and methods to clearly reflect the time reference point.
And to avoid constant conversion, the processManifest method also
attempts to converge to time relative to the start of the window as
quickly as possible.

Issue: #8537
PiperOrigin-RevId: 357001624

RELEASENOTES.md

@@ -8,6 +8,9 @@
     *   Fix a bug where setting playback parameters while using video tunneling
         would cause an error to be thrown
         ([#8570](https://github.com/google/ExoPlayer/issues/8570)).
+*   DASH:
+    *   Fix playback issue for multi-period DASH live streams
+        ([#8537](https://github.com/google/ExoPlayer/issues/8537)).
 *   IMA extension:
     *   Fix handling of repeated ad loads, to avoid ads being discarded if the
         user seeks away and then back to a preloaded postroll (for example).

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/manifest/Period.java

@@ -30,9 +30,7 @@ public class Period {
    */
   @Nullable public final String id;
 
-  /**
-   * The start time of the period in milliseconds.
-   */
+  /** The start time of the period in milliseconds, relative to the start of the manifest. */
   public final long startMs;
 
   /**