Return requested position when selecting tracks in FakeMediaPeriod.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170360487

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaPeriod.java

@@ -81,7 +81,7 @@ public class FakeMediaPeriod implements MediaPeriod {
         streamResetFlags[i] = true;
       }
     }
-    return 0;
+    return positionUs;
   }
 
   @Override