Limit insets adjustment of split bounds in portrait mode

I91d3a430 adjusts insets when calculating divider bar snap positions, to
avoid size-compat mode when switching unresizable apps. However,
limiting the adjustment to portrait mode would be enough since it's for
apps in landscape when they are latterboxed. This also helps align the
middle divider bar position with navigation bar in landscape mode.

Fix: 242918407
Test: switching split position of unresizable apps in landscape won't
      show size-compat mode button
Test: divider bar position aligned with navigation bar in landscape with
      the consideration of insets

Change-Id: I8722393c3f591de2b02098ec279b23c82a7daeb6