Fix fallback for emergency calls

When the first call service disconnects the
disconnect state causes the in-call UI to close.
This means that falling back to the call assistant
for emergency calls didn't work.

Fix was to not set the disconnect state until fallback
was completed.

Bug: 18593307
Change-Id: Ifd0a83662b3edd9b7f324a3eb0b8d519cab44513