Add root information to MtpDatabase.

The CLs adds new table to MtpDatabase that contains additional
information to provide DocumentContract.Roots.COLUMN_*. Client code can
obtain these information by using the newly added queryRoots method.

BUG=25162822
Change-Id: Ib6596d38cbb4a42b689c75fffbd73585c0fb524c
2 files changed