Prevent misuse of getFocusedTaskOnDisplay

This CL makes getFocusedTaskOnDisplay to return only taskId instead of
full TaskInfo whose properties may be stale as this observer only
updates the cache when the focus changes.

Flag: com.android.window.flags.enable_block_non_desktop_display_window_drag_bugfix
Flag: com.android.window.flags.enable_desktop_first_top_fullscreen_bugfix
Bug: 436462692
Test: ShellDesktopStateImplTest
Test: DesktopTasksControllerTest
Change-Id: Ide32a0ac2bed020a922964228ce64c145dbf7dae