Explicitly track consumed state for WindowInsets

Treating 0-insets as fully consumed is incorrect since it means that
you can't dispatch empty insets down the view hierarchy - traversal
terminates immediately. Track consumed state independent of actual
values. Replacing a given set of insets with all zeroes will mark it
consumed.

Bug 15341653

Change-Id: I55b33b7dfbf4cae1e906a82140537156cffdbf47