IMS: Enable CallModify functionality

1. Process CallModify response in CallManager

2. Add isVTModifyAllowed api to Phone

Change-Id: I2bec75befa0a61191565fabe997211b969bab7a8

Conflicts:
	src/java/com/android/internal/telephony/CallManager.java
	src/java/com/android/internal/telephony/Phone.java
	src/java/com/android/internal/telephony/PhoneBase.java
	src/java/com/android/internal/telephony/PhoneProxy.java