Fix janky add guest transition

Delay updating the user switcher until the
switch is complete, so we aren't showing
an intermediate state while the screen is
frozen.

Also pokes the UserInfoController to update
the user avatar while the screen is still frozen
so it is ready by thawing time.

Bug: 22557684
Change-Id: I020b8c1ad42c03ab9162ed384ecfe54ed12f998d