Fix warning about ambiguous `@EnsuresNonNull` annotation

```
[expression.parameter.name.shadows.field] The postcondition
EnsuresNonNull on the declaration of method 'setContentTimeline'
contains ambiguous identifier 'contentTimeline'. Use
"this.contentTimeline" for the field, or "#1" for the formal parameter.
  private void setContentTimeline(Timeline contentTimeline) {
               ^
```

PiperOrigin-RevId: 448285571

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaServerSideAdInsertionMediaSource.java

@@ -605,7 +605,7 @@ public final class ImaServerSideAdInsertionMediaSource extends CompositeMediaSou
   }
 
   @MainThread
-  @EnsuresNonNull("contentTimeline")
+  @EnsuresNonNull("this.contentTimeline")
   private void setContentTimeline(Timeline contentTimeline) {
     if (contentTimeline.equals(this.contentTimeline)) {
       return;