Make MandatoryUpdateMachine use LLLoginInstance's LLNotificationsInterface.

LLLoginInstance has a test hook setNotificationsInterface(), used by
lllogininstance_test.cpp to redirect notifications through a dummy
LLNotificationsInterface implementation. Certain of LLLoginInstance's
MandatoryUpdateMachine state classes need to post notifications too; but until
now they directly called LLNotificationsUtil::add(). In the production viewer,
this should (!) be the same as calling through LLLoginInstance::mNotifications
-- but it broke two of the LLLoginInstance unit tests, so they were skipped.
Since MandatoryUpdateMachine's constructor is already passed the invoking
LLLoginInstance&, make it store the reference. Add MandatoryUpdateMachine::
getNotificationsInterface(), which forwards to new LLLoginInstance::
getNotificationsInterface(). Change LLNotificationsUtil::add() calls in
MandatoryUpdateMachine state classes to call through mMachine's
getNotificationInterface() instead.
This allows us to remove #include "llnotificationsutil.h" from
lllogininstance.cpp, also that #include plus stub LLNotificationsUtil::add()
implementation from lllogininstance_test.cpp.
Finally, it allows us to remove the skip() calls from the two unit tests.