floss: telephony: Support hold call operations

Implemented the following functions and wired to btclient:
- ReleaseHeld
- ReleaseActiveAcceptHeld
- HoldActiveAcceptHeld

Also handled the above commands sent from HF.

Bug: 214149380
Tag: #floss
Test: HFP/AG/ECS/BV-01-I, HFP/AG/ECS/BV-02-I
Change-Id: Iae8e068981a601d23a1bcf1a5457b2d59318662e