Getting image dimensions from objc side

Summary: Getting image height and width from UIImage and sending it over in event. Also, adding allowImagePaste prop in TextInput.js

Test Plan: NA

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D608