Fix NPE when querying for already uninstalled package.

It's possible that by the time StorageManagerService receives
the opChanged() callback, the package whose appop was allowed/denied
has already been uninstalled.

Bug: 133442691
Test: manual
Test: atest android.permission2.cts.RestrictedPermissionsTest
Test: atest cts/hostsidetests/appsecurity/src/android/appsecurity/cts/ExternalStorageHostTest.java
Change-Id: Ia032fe174c8796cafffba22036c2afe974c7dea8