Update FirebaseListAdapter to use Context instead of Activity

[WillieCubed]

Fixes #773.

[WillieCubed]

I know this would be a breaking change, but is there any reason we can't just use LayoutInflater.from(view.getContext()).inflate(mLayout, viewGroup, false); instead of having a dedicated constructor parameter? This would of course be changed in the next major release. (@samtstern)

[samtstern]

@TheCraftKid you're probably right, but given that it's easy to get a Context anywhere you'd want to use ListAdapter I don't know if that's worth the break.