login: Add retrieve primary user to session manager dbus constants

Add retrieve primary user method name to public dbus constants so
it can be used by other packages.

BUG=b:77568272
TEST=cros_workon_make --board=nami system_api --test --install
compiles and builds.

Change-Id: Iaf17df366798df53a8be7544df7acc7cee969418
Signed-off-by: Mengqi Guo <mqg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1198047


Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>