Tidy up naming around "sync disabled" behavior

Tidy up naming around the DeviceConfig "sync disabled" behavior.

Adds "mode" to method names make the behavior more obvious / consistent,
as suggested in the review for http://ag/15083070. Various strings and
constants have been updated to track this.

Some javadoc improvements are included for improved clarity.

This is an internal refactoring only. The command line verbs have been
left as "get_sync_disabled_for_tests" and "set_sync_disabled_for_tests".

Bug: 185786624
Test: treehugger only
Change-Id: I00a472966b0a8e2658a1d3365b9beb9a47c58191