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
* Created b/268120202 to track other states of split
selection that are unhandled on 3P launchers
Test: Able to initiate split in overview with 3P launcher
Fixes: 264576761
Change-Id: I0311fe83380bf914fa79956b2565b61cc5eab916