Auto time should show dynamic subtext when restricted.

Auto time preference should show dynamic subtext corresponding to
the current status of auto time when it's restricted, instead of
hard code "enabled by admin", as it could either be enabled or disabled
when disallow_config_date_time is set.

Bug: 73811933
Test: manually by TestDPC
Change-Id: Ia7181e8a67ccfe72117f95133e9081375c121fe3