AudioService/WireAccessoryManager: change boot completion detection method

BOOT_COMPLETED intent is not a reliable way for system services
to detect boot completion. The intent broadcast can be significantly
delayed and there is no guaranty that system services
receive it before apps.

Use a systemReady() method called by SystemServer instead.

Bug: 14323903.
Change-Id: I781596a3545e7a1e719799982347cbcd9a4c9009