Move FakeRenderer's DrmSession releasing from onReset() to onDisable()

This seems to match DecoderVideoRenderer more closely:
b1e56304a1/library/core/src/main/java/com/google/android/exoplayer2/video/DecoderVideoRenderer.java (L300)

Although MediaCodecRenderer does it in onReset() and then calls that
from onDisable():
b1e56304a1/library/core/src/main/java/com/google/android/exoplayer2/mediacodec/MediaCodecRenderer.java (L754)

PiperOrigin-RevId: 315859212

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

@@ -154,7 +154,7 @@ public class FakeRenderer extends BaseRenderer {
   }
 
   @Override
-  protected void onReset() {
+  protected void onDisabled() {
     if (currentDrmSession != null) {
       currentDrmSession.release(/* eventDispatcher= */ null);
       currentDrmSession = null;