VirtualizedList: Migrate to React.Context

Summary:
Migrates `VirtualizedList` off legacy context by creating `VirtualizedListContext`.

Changelog:
[General][Changed] - Migrated `virtualizedList` legacy context to `React.Context`.

Reviewed By: TheSavior

Differential Revision: D21370882

fbshipit-source-id: 2fa99ee0bc0e6b747a2d3fe7c66ee402c6b9c5af

Author: yungsters