Rewire WallpaperManager keyguard callback

- keyguardGoingAway is not changed (but happens outside WM lock).

- keyguardAppearing is sent if the flag is enabled.

- When the flag is enabled, both commands are only sent if the recipient
  has proper permissions to subscribe to keyguard events.

Test: atest WallpaperManagerServiceTests
Bug: 395897130
Flag: android.app.notify_keyguard_events
Change-Id: I328a1d602be691dd57d10eb41c1679f2b5321d97