Cross, Gregory Llanos, srinu sandy, Isaac Gutierrez, Roberto Billi and also others who pinged me personally on my email id,
I know I am a bit late but yes I made it working using FileManager.
I was not getting enough time to finish this but this time I made it working and published the code in and Github repo also.
Please read it on the below link
Native Android screenshot/image sharing in Unity using FileProvider
GitHub Link: https://github.com/agrawalsuneet/UnitySharingCenter
Please try this and let me know if any of you face any issue.
Cheers guys. Happy Coding :)