Check if downloaded bg sounds are still on server

[harshadmanglani]

An edge case for #147 - whenever downloaded bg sounds are shown in offline mode, there is a possibility that a bg sound on server a month ago was downloaded but now has been removed from the app. However, it would still show up whenever the user opens the app without connectivity.

We need to write a function that checks the downloaded bg sounds' validity every time the user opens the app with connectivity. Primevideo does something similar (yet more complicated) where they set a timer so as to how long downloaded/offline content is valid without connectivity. We can come back to this later.