Use UUIDs for call management in Headset Client (HF)

Using UUIDs that are managed by the service gives more control and error
handling in the service since it has control over assigning them.

Bug: b/29788044
Change-Id: I8483f8e61a33302ba95d544828947d7fb4a21be9
(cherry picked from commit dbeab2c6e12693fe9b06b6a680677da5325c9230)