gril: add call-forwarding support

Add parcel building/parsing functions for call-forwarding support.
rilmodem: clean/re-factor call-forward atom

Clean up the call-forward atom by reducing code
duplication, and re-factoring to use new grilreply/
grilrequest parcel functions.
build: enable rilmodem call-forwarding atom
plugins/ril: enable call-forwarding support