BackHandler doesn't work if bottom sheet active

[sergionettl]

Please complete the following information:

• Library Version: v0.1.5
• Affected Device(s) : Any

Describe the Bug:
Adding BackHandler onBack callback doesn't get called if BackHandler is added and the bottom sheet in visible.

Expected Behavior:
To control the back navigation manually I would expect that BackHandler to work in combination to the FlexibleBottomSheet.

Notes:
I noticed looking at the source code that FlexibleBottomSheetPopup controls the BackHandle behaviour and that it will force you to go to sheetState.intermediatelyExpand() if current is FullyExpanded. What if we want to make it close completely instead?