Update REASON_IMS_ACCESS_BLOCKED definition to match framework change.

This is changed in ag/2484808 to be defined by a constant telecom string. It
will be available to public in O MR. We have to hard code it until then.

(This is a take over of cl/161020406 from wangqi@)

Test: VideoCallNotAvailablePromptTest
PiperOrigin-RevId: 161030078
Change-Id: I0e1d8d98cb1f64b035d9063b28f8ba039c7378e4