Return null value if getActiveAdminUncheckedLocked returns null.

- Split per user version of getUserRestrictions into a separate method
  in DPMS and make the per-user version return null if the admin
  parameter is not a valid one.
- Update isAccessibilityServicePermittedByAdmin and
  isInputMethodPermittedByAdmin to return false if the admin parameter
  is not a valid one.

Bug: 27909087
Change-Id: I6f4cae6552cbfe02dc4a92b04eeeddf0314e0974