Workaround bad base64 being sent by Gerrit

Occasionally Gerrit will return Base64 that is impropperly encoded
 while attempting to download the diff. We can avoid the base64
 fails by downloading the patch in ZIP format. This methodology has
 two benefits.

1) Avoids bad Base64 encoding
2) Smaller network IO as the patch's diff is compressed

PS2: Resolve conflicts

PS3: Use Volley for download; change DiffDialog to use the file path
 rather than a FileInfo object.

Change-Id: Idc0cdf04eacc69d0faf2422cd9af30386cec599a