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

quickstep/src/com/android/launcher3/taskbar/Utilities.java

@@ -16,8 +16,6 @@
 
 package com.android.launcher3.taskbar;
 
-import java.util.StringJoiner;
-
 /**
  * Various utilities shared amongst the Taskbar's classes.
  */
@@ -25,12 +23,6 @@ public final class Utilities {
 
     private Utilities() {}
 
-    static void appendFlag(StringJoiner str, int flags, int flag, String flagName) {
-        if ((flags & flag) != 0) {
-            str.add(flagName);
-        }
-    }
-
     /**
      * Sets drag, long-click, and split selection behavior on 1P and 3P launchers with Taskbar
      */