[Application][D-Bus] Fix crash when shutting down

DBusManager owns the Bus connection and was being destroyed before the
service provider that uses it.

TEST=Loaded xwalk --run-as-service and xwalkctl <APP_ID> then closed the
window. The first process should not crash but exit cleanly.
Merge pull request #1517 from cmarcelo/dbus-shutdown-crash

[Application][D-Bus] Fix crash when shutting down