metrics: Convert Metrics to DBusDaemon

In order to remove glib, convert Metrics from the glib message loop to
DBusDaemon, which is based on base::MessageLoop.  Also use the DBusDaemon's
dbus::Bus object directly to set up the CrashReporter signal handling, as
the ObjectProxy used by chromeos-dbus-bindings requires all signals be sent
from registered name owners.

BUG=chromium:431838
TEST=Unittests and hwtests pass.
TEST=`test_that -b panther <IP> platform_MetricsUploader` passes
CQ-DEPEND=CL:236652

Change-Id: I6bc1f66999a43065b0d48325b031cd36bb782b76
Reviewed-on: https://chromium-review.googlesource.com/234359


Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Steve Fung <stevefung@chromium.org>
Tested-by: Steve Fung <stevefung@chromium.org>