audioserver.rc: start audioserver earlier

We have seen sometimes SystemServer's StartAudioService wait for native
audio policy service.
This CL will move audioserver to be started on "boot" trigger of init.rc
where class 'hal' is started and data/ access are also ready, so it will
give audioserver more time to initialize to avoid service backoff.

Bug: 64218899
Test: 50 reboot cycles on Taimen
Change-Id: Ic220eb74a7a0df64b531a28c7b9d6fb6d38d3699