Reject system user in revokeUserAdmin()

System user should not be removed from admin.

This CL also fixes a typo in a comment in revokeUserAdmin().

Bug: 411194997
Test: atest FrameworksServicesTests \
        --test-filter=".UserManagerTest#testRevokeUserAdmin.*"
Test: Manually test with CL:33082444 as follows:
  $ adb shell cmd user revoke-user-admin 0
  $ adb shell cmd user list -v | grep id=0
  0: id=0, ... flags=ADMIN|INITIALIZED|PRIMARY|SYSTEM (running)
Flag: EXEMPT bug fix

Change-Id: Ibdc1925cbf223958a95bde70ddba4ed8fcf7578c