The CL introduces MAPPING_KEY column to the database and lets Mapper use
the column to map IDs of devices.
It also removes the concept of mapping mode from Mapper for
simplyfing. Now Mapper just tries to multiple mapping keys (MTP
identifier, display name, and mapping key) to find candidate of ID
mapping.
BUG=26212981
Change-Id: I19f6c7dac146047e9978de4eb33d5076406037ad
637a2010f4