Fix OwnersTest when run with -e package

It was because UserManager.get() returns a static cached instance but
we should always be using the mock instance that's created for each test.

Bug 24378326

Change-Id: Id4663e7676d2d0130622055a97fbde0884714349

services/devicepolicy/java/com/android/server/devicepolicy/Owners.java

@@ -101,7 +101,7 @@ class Owners {
 
     public Owners(Context context) {
         mContext = context;
-        mUserManager = UserManager.get(mContext);
+        mUserManager = context.getSystemService(UserManager.class);
     }
 
     /**

services/tests/servicestests/src/com/android/server/devicepolicy/DpmMockContext.java

@@ -218,6 +218,11 @@ public class DpmMockContext extends MockContext {
         throw new UnsupportedOperationException();
     }
 
+    @Override
+    public String getSystemServiceName(Class<?> serviceClass) {
+        return realTestContext.getSystemServiceName(serviceClass);
+    }
+
     @Override
     public PackageManager getPackageManager() {
         return packageManager;