Give adb a proper progress bar.

This factors out the duplication in the single-file progress, and adds a
whole-sync progress percentage to the front of the line. A number that's
both more interesting and easier to read.

This also fixes the >100 percentage reporting for files of unknown size.

Bug: http://b/26189482
Change-Id: I51501ccef6ae1f52425db0eb0862d87e90947a6c