audio_effects HAL uses "default" service name

The getService() and registerAsService() methods of interface objects
now have default parameters of "default" for the service name. HALs
will not have to use any service name unless they want to register
more than one service.

Test: builds; VTS HalAudioEffectHidlTargetBasicTest passes

Bug: 33844934
Change-Id: I5283bf7c2d0645bd38a16e92cab05103c1bcdf26