Make the content view of TestActivity fit system window insets

This CL fixes InputMethodServiceTest which was borken by the edge-to-
edge enforcement because the critical items would be obscured by
system bars. It can be fixed by calling setFitsSystemWindows(true) to
the content view.

Fix: 335382920
Bug: 309578419
Flag: com.android.window.flags.enforce_edge_to_edge
Test: atest FrameworksImeTests:InputMethodServiceTest#testFullScreenMode
Change-Id: Ib6d4adff9be448b4118a5077189c8234c51fa82b