Update object info when writing a file.

The MTP spec does not offer a way to update bytes of exisitng files, so
our provider implementation creates a new file with new bytes and
removes old one.

Previously the new file uses new document ID and the exising document ID
is expired. Also the provider does not update the metadata
database. Thus users see the old flie in DocumentsUI but actually the
files is not accessible.

The CL updates the database with exisitng document ID, so that we can
access the new file with exisiting document ID.

BUG=26549400

Change-Id: I629b707a2e662b34625e8b28857ef818d8933996