[BottomNavigation] Fix hardcoded number of children

[anyercastillo]

Fixes a bug that prevents developers to define an arbitrary number of maximum item in BottomNavigationView.

The problem.
BottomNavigationMenuView was using the static constant BottomNavigationView.MAX_ITEM_COUNT as part of its logic. So overriding BottomNavigationView.getMaxItemCount() with a value greater than BottomNavigationView.MAX_ITEM_COUNT causes the exception ArrayIndexOutOfBondsException

Related to #543

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[imhappi]

lgtm 👍