Convert currentFling to a SharedFlow.

This is fundamentally an event flow. Some tests were receiving currentFling=[FlingInfo] followed 1ms later by currentFling=null, causing the initial fling to be dropped and devices under test to remain locked when they shouldn't have.

This also can't just be a StateFlow, since we do need to know when the fling animation has ended (currentFling=null). We can work on re-architecting this to avoid SharedFlow possibly, but for now, this fix makes the most sense.

Bug: 278086361
Test: presubmit runs, seem to be the only way to trigger the issue reliably
Flag: com.android.systemui.keyguard_wm_state_refactor
Change-Id: I503ce895bf0df3cbc43868a4f2dbef2a827f7a5f