Use Handler callback instead of sleep() to catch InteruptedException.

The sleep used to simulate data load times is ignoring InterruptedExceptions.
(This is intended and in line with SystemClock.sleep()). However, when a
Loader cancels an ongoing load, it uses interrupts. To be able to catch these
and to immediately return from the reading data source, a handler callback
is used instead of the sleep() method which allows interuptable waiting.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=168212652

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

@@ -217,7 +217,7 @@ public class FakeDataSource implements DataSource {
     return dataSpecs;
   }
 
-  protected void onDataRead(int bytesRead) {
+  protected void onDataRead(int bytesRead) throws IOException {
     // Do nothing. Can be overridden.
   }