bootctrl 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; verify HAL still works

In support of b/33844934

Change-Id: I2c0aa199b5649f720710bfd564c077b6ce3acc9c
Merged-In: I43f282ffedf18e4b0817b49fc4860ac39b127d04