Don't change the controlling window to null when focusedApp is also null

This CL considers that the null focusedApp is a temporarily state. So
when mFocusedApp is null, and both mTopFullscreenOpaqueWindowState and
mFocusedWindow are null, updateSystemBarAttributes will be skipped.

Bug: 329124127
Flag: com.android.window.flags.force_show_system_bar_for_bubble
Test: atest InsetsPolicyTest InsetsStateControllerTest
Change-Id: I94cfc6414ef7288883a9305380ded7bf453e2948