Change command-line interface for device_config

Change the command-line interface for device_config:
is_sync_disabled_for_tests -> get_sync_disabled_for_tests. Instead of
printing true or false, it now prints the current mode. This makes it
easier to restore the previous mode after tests.

Bug: 185786624
Test: atest core/tests/coretests/src/android/provider/DeviceConfigTest.java
Change-Id: I712064d879cb25b94c752b3488d169688db4a86d