Fix DOMRpcRouter breakage

https://git.opendaylight.org/gerrit/#/c/74209/ broke the DOMRpcRouter
for UTs that use the empty constructor. The mdsal DOMRpcRouter no longer
implements DOMRpcProviderService directly so the SchemaContext was not
getting set via onGlobalContextUpdated.

Change-Id: Ic9381742f43b05264724eb80eef41201a1285c2a
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>