Rework the device management backend implementation.

Rework the device management backend implementation.

Introduce a service object that lives in the profile that hands out
backends for the clients to use. Living in the profile also allows to
make use of the profile's request context, which has the advantage over
the independent request context implementation that stuff is more
efficient (i.e. we need to only resolve the proxy once).

BUG=none
TEST=device_management_service_(unit|browser)test.cc

Move DeviceManagementPolicyProvider into the profile.

BUG=None
TEST=existing unit tests succeed