Keyboard shortcuts: one instance refactor

Make all the instance public methods private
and add static methods to control the behavior:
* show
* dismiss
* toggle

Make sure we only deal with one instance of the
KeyboardShortcuts (previously both the
KeyboardShortcutsReceiver and BaseStatusBar were
creating instances).

Makes sure the instance is destroyed when dismissing
and when showing it either creates a new one if none
exists or reuses the existing one.

Also fixes an existing nasty issue where in order
to dismiss the dialog from BaseStatusBar we were
always first creating a new instance.

Bug: 28012198
Change-Id: I207553dd45ae535edc64b6292a472fa0899029b0