Provide AllApps item OnLongClickListener through ActivityContext.

This cleans up how alternative AllApps implementations such as Taskbar
and SecondaryDisplay override the default INSTANCE_ALL_APPS listener.
This change will also be helpful for Toast in Taskbar drag-n-drop.

Test: Manual
Bug: 289261756
Flag: n/a
Change-Id: I55eb881bcd1e210852d435a3ea2a6686ce0a9838