service: Add Start|StopScan API to LowEnergyClient

Added the StartScan and StopScan methods to LowEnergyClient and basic unit
tests. This currently only supports regular scans with no batch scan or
hw/sw filters.

Bug: 25744656
Change-Id: Iddf6e897377f90a5eef81ef36696b1d7074a7dab
3 files changed