Ensure only one context menu is shown at a time

Refactors the menu helper classes. Both classes now implement a common
MenuHelper interface, which eliminates the need to keep separate helpers
on PhoneWindow and unifies the DecorView showContextMenuForChild()
implementations.

We now explicitly dismiss any previously shown context menu before showing
a new context menu. Previously we relied on the modal nature of the dialog
context menu to prevent multiple menus from being opened at once, but this
is no longer reliable with popup context menus.

Bug: 25656520
Change-Id: Idab3daa6d6888f803f2e33660fe1dd488e4c28d1