mGerrit: Fix GerritTask not showing transfer progress on some devices

UrlConnection transparently sends "Accept-Encoding":"gzip" by default.
Because the underlying mechanisim has no way of knowing the unzipped
size of the stream it appeared to download more bytes than was returned
by UrlConnection#getContentLength().  On other devices UrlConnection
stripped the value from "Content-Length" header as its inaccurate
while the stream is gzip'd returning -1 and causing the ProgressDialog
to show a generic message instead of showing the bytes as they are
downloaded.

Moved calculations off the main thread to show bt progress updates
quicker... Its still not as fast as possible but much better.

Thanks @razorloves for pointing this error out and spearheading
efforts to resolve the issue.  Razor did all the work here by
finding the docs to support his theory and providing sample
code to fix the issue. Thank you for such on point helping!

Change-Id: Ida506e46a40af2b87eaaf2ba4c01f15a7df380bf
(cherry picked from commit 7bc8fa1a268a997f2ded6bb6a8ae38fbfb3e394d)