Fix testTransientVisibilityOfFixedRotationState

After commit 2bf469b4,
updateSystemBarAttributes won't early return if winCanidate is null. If
DisplayPolicy.mFocusedWindow is null, then the bar control target in
the test will be null as well.

This CL sets the app window as the focused window in DisplayPolicy.

Bug: 329124127
Flag: com.android.window.flags.force_show_system_bar_for_bubble
Test: atest InsetsStateControllerTest
Change-Id: Ibdfb9fcb054eb89666d76839012122912be7a443