[BottomSheetBehavior] Support child's marginTop

[emakar]

Currently BottomSheetBehavior with marginTop behaves badly - child jumps on screen depending on layout phase and appears in different places when showing
Also it's impossible to use both fisToContents and expanded offset. The only workaround is to set marginTop to CoordinatorLayout, however this greatly limits other views in this parent

The solution is to support marginTop as if it were with an expanded offset compatible with fisToContents.
One possible soulution is proposed in #2094

[drchen]

Hi thanks for providing a pull request for fixing this. It looks like a workaround, though. We will try to do some investigation to see if there's a more general solution and go back to the pull request. : )