Desktop screenshots: Guard app window option behind feature flag

Create a new aconfig flag to guard the app window region type in the
pre-capture UI toolbar.

Bug: 427497534
Test: atest PreCaptureViewModelTest
Test: Manually verify UI
Flag: com.android.systemui.desktop_screen_capture_app_window
Change-Id: I41f3caacd707bab58b8e2fd6558c1b6f9de71484