Trim memory when disabling ExtractorSampleSource.

This prevents memory leakage even in the case where the app
is still holding a reference (direct or indirect) to the
Allocator.

library/src/main/java/com/google/android/exoplayer/extractor/ExtractorSampleSource.java

@@ -177,6 +177,7 @@ public class ExtractorSampleSource implements SampleSource, ExtractorOutput, Loa
         loader.cancelLoading();
       } else {
         clearState();
+        bufferPool.trim(0);
       }
     }
   }
@@ -294,6 +295,7 @@ public class ExtractorSampleSource implements SampleSource, ExtractorOutput, Loa
       restartFrom(pendingResetPositionUs);
     } else {
       clearState();
+      bufferPool.trim(0);
     }
   }