Do not reset period uid when DashMediaSource is released

When the source is prepared again after stop, the period uid
is calculated by subtracting the `firstPeriodId` from the
period uid that is passed in to `createPeriod`. When this
happens after stop, the uid from the old period uid that
is still stored and has the value of the last played uid.

Hence the `firstPeriodId` must not be reset when released.

Issue: #10838
PiperOrigin-RevId: 539028570

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

@@ -523,7 +523,7 @@ public MediaPeriod createPeriod(MediaPeriodId id, Allocator allocator, long star
     DrmSessionEventListener.EventDispatcher drmEventDispatcher = createDrmEventDispatcher(id);
     DashMediaPeriod mediaPeriod =
         new DashMediaPeriod(
-            firstPeriodId + periodIndex,
+            /* id= */ firstPeriodId + periodIndex,
             manifest,
             baseUrlExclusionList,
             periodIndex,
@@ -571,7 +571,6 @@ protected void releaseSourceInternal() {
     elapsedRealtimeOffsetMs = C.TIME_UNSET;
     staleManifestReloadAttempt = 0;
     expiredManifestPublishTimeUs = C.TIME_UNSET;
-    firstPeriodId = 0;
     periodsById.clear();
     baseUrlExclusionList.reset();
     drmSessionManager.release();