Toggle taskbar button nav assistant availability

* New signals coming in from Sysui reflect the toggle
in Settings for long pressing on home button to
invoke assistant.

Fixes: 255909545
Test: Manual + added unit test for TaskbarNavButtonController
Change-Id: Ic65a80b0b9697990931b7e89756773fb086cc3bd

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

@@ -320,6 +320,12 @@ public class TaskbarManager {
         }
     }
 
+    public void onLongPressHomeEnabled(boolean assistantLongPressEnabled) {
+        if (mNavButtonController != null) {
+            mNavButtonController.setAssistantLongPressEnabled(assistantLongPressEnabled);
+        }
+    }
+
     /**
      * Sets the flag indicating setup UI is visible
      */