fastbootd: Support TCP protocol.

The current fastbootd only supports USB protocol. But some Android TV
devices are built without USB port. The fastbootd cannot be used on
those ATV devices due to it.
This change adds TCP protocol for such devices and fastbootd.protocol
property is added to control which protocol to use.

BUG: 152544169
Test: manual test.
Change-Id: Idc391e677eb6a1880036419ba5f6c4160e8dbcbc