audiopolicy: refactor audio command threads

- Use strong pointers for command data to avoid transfering
ownership of the object to receiver thread.
This avoids waiting for the command acknowlegement to proceed
with next command on server side.
- Use a separate mutex for each command condition.
- Factor in duplicated code to send commands.

Change-Id: Ib0526e7c4fa64a71ad4015f477c6361727c6f40a