Added option for setting both screens and cropping

[ApoorvaAditya]

Fixes #11

[ApoorvaAditya]

I'm not familiar with Java and Native Android development so there is a bit of code duplication and I am not really sure how to use function overloading properly here so please improve this. I've also not checked for errors when the crop integers are negative or something invalid and how that would affect the package.

[AdityaMulgundkar]

@ApoorvaAditya Thanks a lot for the PR! I have merged your code into master & I'm testing a few things out in the develop branch before pushing it to live on pub.dev. I'll let you know once I've done that.

[AdityaMulgundkar]

@ApoorvaAditya Great job on the implementation! I have pushed your code to live on pub.dev (https://pub.dev/packages/wallpaper_manager), you can check the new features by upgrading/setting the version to 1.0.8.

Thank you so much for this.

[ApoorvaAditya]

@AdityaMulgundkar Thanks for the merge! Glad I could contribute. :)