Prefer use of BackHandler instead of BackAndroid

(1) fix React.PropTypes is deprecated issue (2) trying to fix Module does not exist (react-native-popup-dialog/dist/Type.js) issue

Fix minor typo

I was using your awesome library, great work 👍 while reading docs, i found this little typo. i don't know that whether i should open a pull request for this trivial issue or leave it as it was but for the peace of my mind i should open a pull request for it anyway 😄