Add config to control phone switch

When UNSOL_RADIO_TECH_CHANGED is received, PhoneProxy
calls updatePhoneObject to decide whether to do a phone
switch. This config determines whether to also do a
phone switch when VOICE_REGISTRATION_STATE changes. Default
value is true

Bug: 17394282
Change-Id: Iae6c35992b932923d74c405d443b1e7e98f750c8