Fix companion uses-permission enforcement

Fixes: b/62662686
Test: Call CompanionDeviceManager#associate from app without uses-permission
declaration
Ensure exception is thrown

Call the method from app with the declaration
Ensure no exception is thrown

In debug mode catch the #isCallerSystem call from NotificationManagerService 
(which runs in the system process) and ensure it passes the check
Change-Id: I26e5d2a7f5e63a346d4ab50b9ded1ec7dbc246a5

services/companion/java/com/android/server/companion/CompanionDeviceManagerService.java

@@ -47,6 +47,7 @@ import android.os.IBinder;
 import android.os.IDeviceIdleController;
 import android.os.IInterface;
 import android.os.Parcel;
+import android.os.Process;
 import android.os.RemoteException;
 import android.os.ResultReceiver;
 import android.os.ServiceManager;
@@ -345,7 +346,7 @@ public class CompanionDeviceManagerService extends SystemService implements Bind
     }
 
     private static boolean isCallerSystem() {
-        return getCallingUserId() == UserHandle.USER_SYSTEM;
+        return Binder.getCallingUid() == Process.SYSTEM_UID;
     }
 
     private ServiceConnection createServiceConnection(