Prevent NPE in PlayerNotificationManager.

The app can set the player to null while messages from the player are still
in flight. This may cause NPEs.

Issue:#4238

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196504077

library/ui/build.gradle

@@ -36,6 +36,7 @@ dependencies {
     implementation project(modulePrefix + 'library-core')
     implementation 'com.android.support:support-media-compat:' + supportLibraryVersion
     implementation 'com.android.support:support-annotations:' + supportLibraryVersion
+    compileOnly 'org.checkerframework:checker-qual:' + checkerframeworkVersion
 }
 
 ext {