Use Shadows for permission mocking instead of mocks

[ArnasSmicius]

I'm trying to simplify all the mocks in the tests. I use Shadows to replace our created mocks in CoperTestActivity. The only issue is that I can't find how to replace PermissionChecker.PERMISSION_DENIED_APP_OP with shadows. This result can be returned on devices whose SDK level is below 23. On those devices, there are no runtime permissions. They always get PERMISSION_GRANTED or PERMISSION_DENIED_APP_OP if the user disables permission through app settings. But still this constant means that permission is denied. Sadly, as I debug, Shadows doesn't handle this thing under the hood. It still returns PERMISSION_DENIED on SDK 21. So, https://github.com/vinted/coper/blob/master/library/src/main/java/com/vinted/coper/CoperFragment.kt#L240-L241 this place is not tested that well in this case. But I'm not sure if it's worth writing our own mocks just to test this scenario. What do you think?

[neworld]

This result can be returned on devices whose SDK level is below 23. On those devices, there are no runtime permissions. They always get PERMISSION_GRANTED or PERMISSION_DENIED_APP_OP if the user disables permission through app settings.

Nowadays it is safe to assume that most of app development happens in >23 minSdk. So I recommend just bump minSdk. It is not very good idea to release not well tested functionality anyways.