Recovery: check adb auth on sideload [1/2]

This makes adbd advertise, using the "service.adb.online"
property that it is online.

One could inspect that property to check if there is a host
device online.

Patch Set 2: New behaviour only if running on recovery mode

Change-Id: I60ac6153d6d40570159575029f50af9d5afa4a25