Fixed Flow typing of TextInput refs

Summary:
`React.ElementRef<HostComponent<mixed>>` is an inexact object type, which can not be spread into an exact object type, as is happening here. This error is masked in types-first mode, but causes the instance type of this component to be `any`. In a future version of Flow, this issue will be fixed, so this change unblocks upgrading Flow.

This change is likely to cause code using `TextInput` refs to find errors which were missed before.

Changelog:
[General][Fixed] - Fixed Flow typing of TextInput refs

Reviewed By: yungsters

Differential Revision: D26733314

fbshipit-source-id: 8aa26ce5b49357b279f76dd1767a17a9fb4dd4f1