Summary:Using TouchableNativeFeedback has been a problem for me because the ripples it makes don't follow the child view's border radii so the ripples stick out of the child view's rounded corners. This PR should fix this problem with a minor caveat: this only works for TouchableNativeFeedback.Ripple and not TouchableNativeFeedback.SelectableBackground. I searched how I could apply corner radius to selectableItemBackground and it doesn't seem to be possible (the prevalent advice is to create the ripple manually which is equivalent to using TNF.Ripple in our case), though I could be wrong.
I added [an example to UIExplorer (TouchableExample)](http://i.imgur.com/CHY9xjW.png). This is my first PR to this repo so let me know if something's wrong. Cheers!
Closes https://github.com/facebook/react-native/pull/6515
Differential Revision: D3126513
Pulled By: mkonicek
fb-gh-sync-id: 1d3e92243abf9706132ae47c485d9e04a9b47d81
fbshipit-source-id: 1d3e92243abf9706132ae47c485d9e04a9b47d81
e438954938
