Reset WindowMagnification when the density is changed

To prevent MirrorSurfaceView not cleared, when the
denisty is changed, we should call
deleteWindowMagnification and enableWindowMagnification
to reset WindowMagnification instead of just removing
the mirrorview.

Bug: 169149030
Test: atest WindowMagnificationControllerTest
Change-Id: Ie1da5f3556cf23482c562b479f483783e09ded1f