Fix issue #129936402: Runtime restart occurs when suspending an app...

NPE at com.android.server.appop.AppOpsService.notifyOpChanged(AppOpsService.java:1431)

Wasn't checking for a null callback list, and also fixed threading
issues.

Fixes: 129936402
Test: Manual
Change-Id: I9c39429421643ebc10e78679c2bec52827603da1