Remove race condition due to RAT technology change.

On RAT technology change old phone is destroyed and new phone created.
Original code performed unregistration of old phone and then registration
of new phone - small window where CallManager.getDefaultPhone() returned NULL.

Code changes order to make sure new phone is registered before old phone
is unregistered.

Bug: 17347478
Change-Id: Ife5b995de90bb5f8fab51af89f7b9aa50e7ec306