Exclude stable insets from task config

When a app is in fullscreen, we exclude navigation bar and status bar
size when calculating the config. However, when in multi-window, and
the task was almost fullscreen, the height/width reported to the app
was actually larger than when it was in fullscreen. In order to fix
this, exclude the stable insets when calculating the task
configuration, and also fix a bug when calculating the screen layout.

Change-Id: I843ae012fb3050c79643d125550aacb6e73d27da