A brave new world for window insets (9/n)

Implement WindowInsets.get(Max)Insets(int typeMask) to allow the
developer to query the inset by inset type.

Also rework InsetsState.calculateInsets to actually construct the
WindowInsets instance that contains this information.

Test: InsetStateTests
Bug: 118118435
Change-Id: Ie316e074c020bdb9808c11608812dea572c8de5d