service: Implement IBluetoothLowEnergy client registration

This CL adds support for the IBluetoothLowEnergy registerClient and
unregisterClient methods. A lot of the code added here deals with
asynchronous callback registration and can be generalized to other
interfaces that asynchronously associate and return client
interface IDs with callback binders.

Bug: 23793954
Change-Id: I82cf26e194c456f04363fe66f094bc803a5aa436
2 files changed