By default no modem is allowed.
Property "bluez5.hfphsp-backend-native-modem" can be 'none', 'any' or the
modem device string has found in 'Device' property of
org.freedesktop.ModemManager1.Modem interface, e.g. for PinePhone
"/sys/devices/platform/soc/1c1b000.usb/usb2/2-1".
This allows the HFP HF to retrieve the list of calls with their index,
state and remote number (if available).
This commit shared the list of calls between modemmanager.c and
backend-native.c, and switches call state storage from ModemManager
states to CLCC states
Allow to answer, reject or terminate a call.
Answering or rejecting a call can only be done on an incoming call.
Terminating a call can only be done on active, dialing or alerting call.
The Voice object lists the Call objects, which provides status of each call.
+CIND call indicator is set if at least one of the call is active.
+CIND callsetup indicator is set if one of the call is in ringing in or out
or dialing state.
The Modem object provides the own number (used by +CNUM), the network
service availability and signal strength (used for the +CIND).
+CIND indicators can be activated/deactivated using AT+BIA except for
call, callsetup and callheld indicators which should always reported.
All indicators are enabled on connection.
