Use precondition isSystemUid DPMS

* Replace enforceSystemCaller and
  isCallerWithSystemUid with
  precondition check.

Bug: 167960209
Test: atest com.android.server.devicepolicy.DevicePolicyManagerTest
Change-Id: Iadda47cdac89bf880b42d85c6645cc4d713797a7
1 file changed