Position bottom bar widgets relative to window (#14406)

Note: also fixes a bug wherein bottom media padding was applied even in the
presence of persistent footer buttons.

The material spec states that the keyboard should be positioned on top
of any bottom navigation bar or persistent footer buttons widget(s).

We no longer inset the bottom of bottom navigation bars / persistent
footer buttons by the bottom viewInset.

Body content bottom (and the bottom of bottom sheets) is now determined
by the greater of:
  1. bottom view inset (the keyboard height)
  2. bottom elements (nav bar, footer buttons)
relative to the window max-Y.