Initial implementation of BottomAppBar (#14713)

* add a FAB NotchMaker to ScaffoldGeometry

* add a notchMaker to FloatingActionButton

* Initial implementation of BottomAppBar

Mainly includes the notch making logic.
Not yet tested as currently there is no way to make the FAB and the BAB
overlap, once #14368 to lands we could add unit tests to the
BottomAppBar as well.

* use a closeable for clearing the FAB notchmaker