Catch exceptions when creating constants for sd card directories

[chrusart]

Setting SDCardApplicationDir constant throws sometimes NullPointerException.

Additionally SDCardDir and SDCardApplicationDir are set once on module load, but external storage can be ejected, unmounted and paths could be not valid anymore later.

This PR:

1. catches NullPointerException (and other exceptions) and sets then empty path (other ideas how to handle exception are welcome) which should be handled in JS then.
2. introduces getSDCardDir and getSDCardApplicationDir methods to get this paths when needed from JS.
3. adds deprecation warning when constants are used.

QUESTION TO MAINTAINERS:
I've read contributing guidelines just now when was about to create PR, should I split it to separate PRs, to bug fix and feature branches. Somehow all this PR changes could or should be merged together maybe. And this is all about Android only and not sure to do all this library testing and use dev repository.

[sivakumar-cf]

Please merge this PR. Our production app breaking this, needed asap.

[chrusart]

There is a 'RNFB MAINTAINER GONE MISSING' notice, it can take some time to merge.
Feel free to use the branch for now.

[bdipak]

@chrusart any idea how much time it will take ? Looking forward to have this change released to npm.

[chrusart]

No idea, I'm not related to maintainers.

[chrusart]

We use our branch in production now, we don't get this crashes anymore and also some iOS crashes fixed:
"react-native-fetch-blob": "github:flatfox-ag/react-native-fetch-blob#e46f94e2ce7f135f3e9088c95bbd53cd79481ab5",

[lll000111]

FYI

react-native-fetch-blob has a new maintained location. Check out the README.

Please consider submitting your PR there, this repository is unmaintained.

NOTE: Patches should be against the current development branch, not against master.