Add optional debug logging to on flag changes

FlagDebugUtils.formatFlagChange() utility to always write the set of
updated flags, with a list of actual changes applied. Examples:

[allow_gesture|device_dozing] +[device_dozing]
[] -[state_started]

Additionally, moved the appendFlag utility to the new FlagDebugUtils

Test: manually verifed the output in logcat
Bug: 261418621
Change-Id: Ie4f2cfcd4b34f0a816db7845e1df4331babed07a