DO NOT MERGE Add support for background scanning

Background scanning (preferred network offload) allows us the
host to sleep while the dongle monitors the presence of any
preferred networks. This allows us for significant power savings
since the system does not have to wake up often to initiate
a scan

Bug: 3453021
Change-Id: I73dcf4e1c9ab4de280c40b6df6147a74ac003123