Create observer flow for Caption handle state changes

Create a flow that observes changes to caption handle state.
This flow is used to trigger app handle education.
Refer to go/one-pager-caption-handle-repository for more details.

Fixes: 361038716
Fixes: 365054334
Fixes: 365926285
Test: atest WindowDecorCaptionHandleRepositoryTest
Test: atest DesktopModeWindowDecorationTests
Flag: com.android.window.flags.enable_desktop_windowing_app_handle_education
Change-Id: I7bfda0d0393de8069bb88882a56cda88ae10caab