Suppress warnings in preparation for Checker Framework 3.12.0 upgrade.

LSC: go/checker-lsc

Tested:
    Sample tests for this CL passed, but some tests failed during the TGP run. Test failures are believed to be unrelated to this CL
PiperOrigin-RevId: 366804637

library/core/src/main/java/com/google/android/exoplayer2/drm/DefaultDrmSessionManager.java

@@ -483,6 +483,8 @@ public class DefaultDrmSessionManager implements DrmSessionManager {
     exoMediaDrm = null;
   }
 
+  // precondition of preacquiredSessionReference.acquire is not satisfied.
+  @SuppressWarnings("nullness:contracts.precondition.not.satisfied")
   @Override
   public DrmSessionReference preacquireSession(
       Looper playbackLooper,