[refactor] Doubled "Container" component

[mateuszmazurek]

I created issue #1135 but it has been "automatically closed because it does not follow the issue template.". There's no adequate issue template for my case so I will paste it here.

I found one confusing thing while working on some feature (it will allow even more customization) for this lib.

We have style prop which can be used to apply additional styling "at the sheet container" (as docs says). Inside BottomSheet component you use internal BottomSheetContainer component which can be styled using undocumented (but typed) containerStyle prop.

Both props are used to style container but:

1. it's hard to understand which container is which one when reading this library code
2. it'll be very challenging to document containerStyle prop so as not to mislead this library users ("two containers? wtf?")

I need to name this "public" container (this styled using style prop, raw Animated.View, child of current BottomSheetContainer) for the purpose of feature I work on but I'm not convinced by any of my ideas. I'm thinking of renaming BottomSheetContainer to BottomSheetRoot (edited) BottomSheetRootContainer (or something similar) and use BottomSheetContainer name for the "container" mentioned in docs (this "public" one). Is it good direction? What do you think @gorhom?