Add API to allow launcher to suspend AutoHideController

* Make suspend/resume methods public
* TaskbarDelegate acts as the AutoHideUiElement
for the purposes of simplicity (vs having Taskbar
itself in launcher implement the interface).
* We re-use existing mNavBar setter in AutoHideController
since we know when tablet is showing there is no
navigation bar present

Bug: 193938507
Test: Taskbar shows when requesting auto-hide suspend.
Change-Id: I36429b4ea357646cf6b6dac7a048905b1986be16