[RESTRICT AUTOMERGE] Launch activities in fullscreen in AssistantStackTests

In this test, activities need to be launched in a fullscreen windowing
mode. Without explicitly specifying it, this test fails in default
freeform devices, such as ARC++.

This change fixes the issue by specifying launch windowing mode.

Bug: 168770671
Test: android.server.wm.AssistantStackTests on eve and crosshatch
Change-Id: I9a5b7307bdfada536983662adced6d33f7aa0f49
Merged-In: I9a5b7307bdfada536983662adced6d33f7aa0f49
3 files changed