Fix bug in policy size tracking

Removing policies that were set before the policy size tracking flag was
enabled causes an NPE as we assume the admin always exist in the policy
size map. Also added admin policy size to dumpsys.

Bug: 336196915
Test: btest android.devicepolicy.cts.DevicePolicyManagerTest
Test: manual testing - set policy with flag disabled, enable flag,
remove profile
Change-Id: I5e9e11c8e45a1733c2b1dcb8b2057a2ac4aedfdf