Fix for #17348, Scrollable failing due to uglify-es
Fix for #17348, Scrollable failing due to uglify-es

Summary:
Fixes https://github.com/facebook/react-native/issues/17348 - Unhandled JS Exception: TypeError: undefined is not an object (evaluating 'this._subscribableSubscriptions.forEach')

Same existing tests applies, just added extra checks.

This patch fixes the issue https://github.com/facebook/react-native/issues/17348

 [ANDROID] [BUGFIX] [Subscribable] - Fix for https://github.com/facebook/react-native/issues/17348
 [IOS] [BUGFIX] [Subscribable] - Fix for https://github.com/facebook/react-native/issues/17348
Closes https://github.com/facebook/react-native/pull/17463

Reviewed By: sahrens

Differential Revision: D7062101

Pulled By: TheSavior

fbshipit-source-id: 1306f4695ffc8748f674de70740ded09e1e390f7

hoangpham95/react-native,javache/react-native,arthuralee/react-native,janicduplessis/react-native,hammerandchisel/react-native,facebook/react-native,janicduplessis/react-native,hoangpham95/react-native,myntra/react-native,myntra/react-native,exponent/react-native,pandiaraj44/react-native,exponent/react-native,exponentjs/react-native,arthuralee/react-native,javache/react-native,javache/react-native,hoastoolshop/react-native,exponentjs/react-native,hoastoolshop/react-native,facebook/react-native,exponentjs/react-native,pandiaraj44/react-native,hoangpham95/react-native,hammerandchisel/react-native,myntra/react-native,hoangpham95/react-native,hoangpham95/react-native,janicduplessis/react-native,pandiaraj44/react-native,hammerandchisel/react-native,hoastoolshop/react-native,hoastoolshop/react-native,myntra/react-native,facebook/react-native,arthuralee/react-native,facebook/react-native,pandiaraj44/react-native,exponentjs/react-native,myntra/react-native,janicduplessis/react-native,pandiaraj44/react-native,hammerandchisel/react-native,hammerandchisel/react-native,exponent/react-native,facebook/react-native,myntra/react-native,javache/react-native,javache/react-native,hammerandchisel/react-native,hammerandchisel/react-native,myntra/react-native,hoangpham95/react-native,pandiaraj44/react-native,pandiaraj44/react-native,javache/react-native,exponent/react-native,exponentjs/react-native,janicduplessis/react-native,exponentjs/react-native,exponent/react-native,myntra/react-native,exponent/react-native,hoangpham95/react-native,exponent/react-native,janicduplessis/react-native,facebook/react-native,exponentjs/react-native,arthuralee/react-native,myntra/react-native,facebook/react-native,facebook/react-native,facebook/react-native,hoastoolshop/react-native,janicduplessis/react-native,janicduplessis/react-native,javache/react-native,javache/react-native,hoangpham95/react-native,arthuralee/react-native,exponent/react-native,hoastoolshop/react-native,exponentjs/react-native,javache/react-native,hoastoolshop/react-native,hammerandchisel/react-native,hoastoolshop/react-native,pandiaraj44/react-native