Review logic to show notification about Bitkit running in BG

[ovitrif]

See #124 (review)

Having the notification show up without wallet info, (ie. on fresh install with no wallet or after wallet wipe) feels a bit strange from the user’s perspective IMHO. Maybe we should look if there’s an optimal solution to run & stop it in connection to whether there is a wallet or not.

[jvsena42]

This can be solved with a method in the MainViewmodel that calls walletExists() and trigger an event to MainActivity