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 457639426db7441cc7a5ad9990fd56ec79b2b003)
(cherry picked from commit b61166fb)