Disable call blocking after emergency call.

Will be disabled for 48 hours after an emergency call.

+ Add utility for noting last time of an emergency call.
+ Add utility for comparing current time to last time of an
emergency call.
+ If current time is within 48 hours of emergency call, display
text in settings explaining why call blocking is disabled.

Bug: 24418319
Change-Id: I6824340f04fae17b73f17843a9f6ef991afa0327