Do not assume DisplayNotificationController is thread-safe

This CL makes WM to use DisplayNotificationController thread-safely by
- changes to call dispatchDesktopModeEligibleChanged with the
global lock.
- hold the global lock to [un]register listeners

Flag: com.android.server.display.feature.flags.enable_display_content_mode_management
Test: ActivityTaskManagerServiceTests
Bug: 406462760
Change-Id: I5ac1b2a232cef14b820407bb4ec5dcc005466ae7