Fix bug where mw landscape cell size is calculated incorretly.

The problem is that the height we pass to the multi-window DeviceProfile
includes the top insets, so we tell widgets that the cell size height is
larger than it actually is.

We use Display#getSize to pass in the size of the window in multi-window
mode. The docs say "the returned size may be adjusted to exclude
certain system decor elememts."

To ensure that system decor elements is always excluded, we take the
minimum sizes of the main profile and it's multi-window variant -- since
the multi-window DeviceProfile sizes are never going to be larger.

Bug: 64851168
Change-Id: I274f838c54f3d01d43dc51e0ff921a6124e12cf7