Don't change the controlling window when the new one is not ready yet

Previously, we allow the system UI controlling window to be null. That
was for stopping windows in non-fill-screen windowing mode controlling
system bar insets. However, when there is no multi-window mode involved,
both mFocusedWindow and mTopFullscreenOpaqueWindowState can still be
null if the next candidate window is not ready. In this case, we don't
change the system UI controlling window as we did before.

Bug: 329124127
Flag: com.android.window.flags.force_show_system_bar_for_bubble
Test: atest InsetsPolicyTest InsetsStateControllerTest
Change-Id: I6b672ab45e02dadef6514bc2abc63d079b8891d9