Move MediaSource masking code into separate class.

The masking logic for unprepared MediaSources is currently part of
ConcatanatingMediaSource. Moving it to its own class nicely separates the
code responsibilities and allows reuse.

PiperOrigin-RevId: 256360904

library/core/src/test/java/com/google/android/exoplayer2/source/ConcatenatingMediaSourceTest.java

@@ -279,13 +279,6 @@ public final class ConcatenatingMediaSourceTest {
     CountDownLatch preparedCondition = testRunner.preparePeriod(lazyPeriod, 0);
     assertThat(preparedCondition.getCount()).isEqualTo(1);
 
-    // Assert that a second period can also be created and released without problems.
-    MediaPeriod secondLazyPeriod =
-        testRunner.createPeriod(
-            new MediaPeriodId(
-                timeline.getUidOfPeriod(/* periodIndex= */ 0), /* windowSequenceNumber= */ 0));
-    testRunner.releasePeriod(secondLazyPeriod);
-
     // Trigger source info refresh for lazy media source. Assert that now all information is
     // available again and the previously created period now also finished preparing.
     testRunner.runOnPlaybackThread(