Fix UploadService unittests.

The unittest was creating a brillo::Daemon in a smart pointer and then
resetting it to a newly created different brillo::Daemon instance. This
creates two brillo::Daemon instances for a short period of time, which
both setup global context (the default message loop).

This patch fixes this situation by deleting the service before creating
a new one.

Bug: 27121855
TEST=`/data/nativetest/metricsd_tests/metricsd_tests` on edison-eng

Change-Id: I3b60cbfcdba77febbfa6104aa0ecad60a29afec5