rework main activity fab show/hide to slide up/down

rework main activity fab show/hide to slide up/down

this is inspired by the HideBottomViewOnScrollBehaviour, which cannot
be used directly, since it doesn't work for showing the fab when the
recycler view is not tall enough to cause scrolling

to reproduce the problem, make the fab hide in the transaction list,
then switch to the account tree, which needs to be collapsed, and try
to show the fab