Desktop screenshots: Hide UI before triggering screenshot

Finishing the ScreenCaptureActivity is not guaranteed to happen before
the screenshot is taken. To workaround this, hide the UI completely
before taking the screenshot, which guarantees the pre-capture UI is
excluded from the screenshot.

Bug: 427500006
Test: atest PreCaptureViewModelTest
Test: Manual
Flag: com.android.systemui.desktop_screen_capture
Change-Id: Ia9a828c323b5dcb1516bcb10bbfa141c860b5572