Keyboard shortcuts: dismiss when activities start via shortcut

* introduced a new intent DISMISS_KEYBOARD_SHORTCUTS and
and new public API in Activity (which sends a broadcast
to KeyboardShortcutsReceiver) which applications can
use to dismiss the keyboard shortcuts.

* plumbing and implementation for a new call to dismiss
keyboard shortcuts from PhoneWindowManager and used it:
** when starting activities invoked via Search+key
** when starting activities invoked via META
** when starting activities via application launch keys

* removed unused variable in
Activity#onProvideKeyboardShortcuts

Note that for apps started via touch (aka non-shortcut)
like tapping the Settings gear icon from the notification
bar the menu is not automatically dismissed.

Bug: 28012198
Change-Id: I83a8d4f342bb8a08115a648648834d0d2bac19fd