Fix visible mirror window after switching user

The window magnification will be set to null if the new user
doesn't request this feature. However all existed mirror windows
are not cleared. We disable all window magnification if the connection
is going to set to null.

Bug: 154077487
Test: manual test
      case 1
      1. Turn on magnification and show the mirror window
      2. Switch to another user and observe it
      case 2
      1. Turn on magnification and show the mirror window
      2. Switch to another user whose magnification is turned on.
      3. Observe if the window is gone.

Test: atest WindowMagnificationManagerTest
Change-Id: I1cf689f22587077258b857e2a114edd93a9fe1e0