Change SplashscreenTests for freeform first devices

This CL does the following 2 things:

* It only verifies top/bottom insets when the activity is in fullscreen
mode and they exist
* Add freeform variants of tests for devices that support freeform
windows

Bug: 196872611
Test: SplashscreenTests on ARC++ S only fails for an ARC++ specific
issue.

Change-Id: Icf90b1742badae0791f3c677aa83dbbc49f3522d