mw.ViewPageTarget: Emit position event after toolbar is animated

In target#setUpSurface, both target#setUpToolbar and
target.surface#initialize are called. #setUpToolbar does an
asynchronous animation.

After that animation is completed we call target.toolbar.initialize
and target.surface.context.update.

Right now ce.ProtectedNode needs to update the position of its
shields when the CE Surface changes position (which it does when
the UI Surface changes position because of the UI Toolbar changing
position), and does so by listening to toolbarPosition.

Adding this event to allow it to listen to that instead.

Change-Id: I826986794630c04c34cef6da36ccb15ff7dde49a
Merge "mw.ViewPageTarget: Emit position event after toolbar is animated"