Make FRP APIs callable by Settings

Background
* If the device is an organization-owned managed
  profile device and a FRP policy is set, the
  factory reset protection data is no longer
  erased from factory reset in Settings.

Changes
* Added isNotEmpty method to FRP policy.
* Allow Settings to call
  getFactoryResetProtectionPolicy
  by checking for the MASTER_CLEAR permission.

Bug: 148847767
Test: manual testing
      atest com.android.server.devicepolicy.DevicePolicyManagerTest
Change-Id: I04f178255dd215579087c33b675b40eed7a6eac7