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