Add device_config cmds for disabling syncs

This adds a "sync disabled" mode which disables bulk updates to
device_config (AKA server flags). This is intended for use during
automated and manual tests that use device_config settings to set the
device into specific states for tests. Without this, devices can sync at
an arbitrary point during a test which can undo device_config changes
the tests have made and cause them to fail / flake.

This mechanism is independent of the mechanism used to sync, thereby
making it suitable for use in CTS or other AOSP tests, i.e. to disable
sync regardless of whether GMS core or an alternative is handling the
sync.

Test: atest core/tests/coretests/src/android/provider/DeviceConfigTest.java
Bug: 185786624
Change-Id: Icd0ce798642eb136dc8b9b1a58a4ecbc6212fdba