Clear caller identity before calls to PlatformCompat

This is a pre-emptive fix for http://b/142942524 before the compat
change permissions are re-added.

Bug: 142942524
Test: atest DevicePolicyManagerTest
Test:
      1. Install TestDPC via adb
      2. Invoke "Set up Te..." from the launcher to create a work profile and follow "next" button and select "skip adding an account"
      3. Select TestDPC from the "work" tab of the list of all apps in launcher.
      4. Using built-in search look for "Password constraints" and tap it.
      5. tap "Minimum password length" and try to set it to 5.
      Before change: Neither user 1010193 nor current process has android.permission.INTERACT_ACROSS_USERS.
      After Change: password quality should be at least 131072 for setPasswordMinimumLength

Change-Id: I026b15f0c2562293402bb8de55b0c63a4bb22a85