In “Causeway,” Jennifer Lawrence and Brian Tyree Henry play a pair of unlikely friends who share a bond forged over mutual loneliness. It’s a quiet, low-key drama that doesn’t announce itself in any particular way, but has a way of sneaking up on you before you realize it. Lawrence is Lynsey, back home in New Orleans after suffering a brain injury while deployed in Afghanistan. Her goal is to rehabilitate as quickly as possible, get a doctor’s clearance and return to the military, where she feels useful and things make sense. Things don’t make sense in New Orleans, where she has a fractured re…