User switcher dialog morphs again.

The refactor dropped support for the dialog "morphing" animation that's
suppored by the pre-refactor code. This CL adds it back.

Essentially, we just pass the DialogShower to the coordinator through
the dialog showing request.

Fix: 254527458
Test: manually verified that the dialog morphs exactly the same whether
the refactor flags are on or off.

Change-Id: If382c2e7afab7063aa252510f6a280a31c755eff