Remove mixins from ScrollView (#22374)

Summary:
As a part of #22301 it turned out that we need to first convert `ScrollView` to class component. As a first step to do so, here's removal of using `mixins` API, in favor of populating `_scrollResponder` field with `ScrollResponder.Mixin` (still used) methods.
Pull Request resolved: #22374

Reviewed By: TheSavior

Differential Revision: D13307775

Pulled By: RSNara

fbshipit-source-id: 16be1df8a0bf9ccc5cc32f3a017a1279f99268ed