Made severe and extreme alert configuration dependency configurable

Before when extreme alert is disabled, severe alert will be disabled
by default because we assume users won't be interested in severe
alert if he is even not interested in extreme (higher level) alert.

Now carriers are allowed to turn this feature off by carrier config.

bug: 29369591
Change-Id: I3eb8a0c3e2df420ea75a77ec01cc447013434219