As I've already worked for a company developing android applications, I did implement a similar video downloader. You can contact me at +91 9701247811 in order to know about my previous work.
Relevant Skills and Experience
I developed three applications which helped the distributors of the company Azista to keep track of the sales and stock details of their products along with real time monitoring of the employees.
Proposed Milestones
$111 USD - When provided with a link of a video from Youtube, Dailymotion, gives a video file.
$111 USD - When provided with a link of a video from Facebook, Vine, Livelea, gives a video file.
I don't think we can have videos from Instagram as we can't have any link as a reference to use as an input for the application. Rest can be done. Will be excited to find out a way to download videos from Instagram.