Import ssh key file browser on Android 12 only shows some file types

[maks]

Following on from #601 the current "import key" MGit's file browser UI on Android 12 (possibly other recent versions too) seems to only display files with known media extensions, eg. mp3, png, wav, etc. Which makes it harder than it needs to be to import a ssh private key into MGit.

This is likely due to security restrictions, so probably need to switch to using a standard Android File browser/selection api.

[EverThingy]

I can confirm that this issue is also present on Android 11. (OnePlus Nord running Oxygen OS).

[ilippert]

I suggest, for the meantime, you reference this issue at https://github.com/maks/MGit/wiki/UserManual#private-key-authentication - because otherwise users might be confused (that is, until they find this issue)

[maks]

Fixed by #616