Add document type to MtpDatabase.

Currently all document roots represent MTP storage, but we will not have
this assumption after starting show MTP devices (not storage) as
document roots.

To distinguish these two types of roots, the CL adds document type
column to the document table in MtpDatabase.

BUG=26120019

Change-Id: I6ee930008aea0b43c1c42b21a198b07eccbd443d