Reduce cutoff size for multi-window layouts to 500dp.

Hammerhead devices have a height of 544dp, which currently means they
will be adopting the layout of the multi-window UI. Change the threshold
so that Hammerhead uses the full-screen layout.
Note that this does not affect NYC because Hammerhead is not supported,
but will be necessary for unbundled dialer.

Bug: 27173087
Change-Id: I6250de91b6e0c5c197d599422ae7a74c27ed9628