floss: telephony: Implemented basic phone operations

Implemented the following functions and wired to btclient:
- IncomingCall
- DialingCall
- AnswerCall
- HangupCall

Handled the following command sent from the HF:
- ATA (answer call)
- AT+CHUP (hangup call)

Since the media stack modifies the phone state as well, added
SetPhoneOpsEnabled for switching the current owner of the phone state
between the telephony and the media stack.

Bug: 214149380
Tag: #floss
Test: HFP/AG/ICA/BV-06-I, HFP/AG/CIT/BV-01-I
Change-Id: I5dfddf699a59d314e9fe40b2fcf7edf641e13288