A brave new world for window insets (8/n)

WindowInsets now keeps track of all insets per type. Insets are
non-additive, i.e. every inset starts out relative to the window
edge, so the IME inset would include the navigation bar inset, but
not vice-versa.

We remove decorWindowInsets because it wasn't used at all.

For compatibility, we map the constructor where we pass in a Rect
to TOP_BAR. This is fine as every query to systemWindowInsets
stableInsets will include this type, so we don't need the
information where it came from.

Test: WindowInsetTest
Bug: 118118435
Change-Id: I1cb37d328060293f9a876e61d4a09e6675fa7197