Extract assistant logic from NavigationBar to share with Taskbar

* Added logic into already existing class used
for A11y, renamed that to NavbarHelper
* Prevent destory() in TaskbarDelegate from being called
without init() call first

Bug: 204273914
Test: Assistant is invokable on launcher unfolded
in gesture nav

Change-Id: Ic3232f375c68fd444c7378bc8ddd5a9b6991bd69