DO NOT MERGE ANYWHERE Bring back support for legacy bt_config.xml

An earlier CL introducing backup support for bt_config.conf blew up
support for legacy file, making upgrading from L-based OS to M-based OS
lose its bluetooth config.

BUG=27824878

Change-Id: Ic23ef2a3559b04224278edffdc0e9380c2760638
1 file changed