Use device key to map device documents.

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