Make the log file names predictable

The problem with google-glog is that the log file names are unpredictable. This makes debugging quite difficult. Here's my use case:

For the system-image integration tests, I want to be able to intersperse s-i logging output with u-d-m logging output so I can more easily associate u-d-m debugging with s-i debugging. Under the old logging regime, I simply opened ~/.cache/ubuntu-download-manager/ubuntu-download-manager.log in append mode, and wrote a header/trailer around each of my test invocations. Everything in between these markers was u-d-m output.

With google-glog, this is impossible, because s-i cannot predict which log file u-d-m's logging output will go into. This makes it practically impossible to synchronize s-i output with u-d-m output to figure out what's going wrong.