Fix memory leak in RCTSafeAreaView

Summary:
According to the retain code collector, RCTSafeAreaView is in a cycle:
-> _bridge -> RCTCxxBridge -> _moduleDataByName -> __NSDictionaryM -> RCTModuleData -> _instance -> RCTUIManager -> _viewRegistry -> __NSDictionaryM -> RCTSafeAreaView
This should break the cycle.

Reviewed By: shergin

Differential Revision: D6213668

fbshipit-source-id: efb9c1dd148b72f66fe4485b81c16cd4c2d18b17