Added mising sendToTarget message for dismissing docked stack

Toast that an app doesn't support split screen wasn't displaying because
of missing line to actually send the
NOTIFY_ACTIVITY_DISMISSING_DOCKED_STACK_MSG message.

Test: Dock an app and the launch an other app that doesn't support
split-screen and make sure the "doesn't support split-screen" toast
shows up.
Test: go/wm-smoke
Fixes: 62281412

Change-Id: Ia2cb3085b6c6bfd27cf9e825315b8ddc89cf5bcd