service: Add advertising support to LowEnergyClient

This CL adds the StartAdvertising and StopAdvertising APIs to
LowEnergyClient. These internally call into the multi-advertising
APIs in HAL and report success or failure in an async callback.
The sequence of calls to enable advertising and setting advertising
and scan response data are serialized internally.

Bug: 23793954
Change-Id: Id093a98f3baca6328653b986857a064e8a722690
5 files changed