Window magnifier state is incorrect after screen is off

The root cause is that the enabled state is not updated when
screen is off.
So correct to use disableWindowMagnifier(displayId, false) to
disable window magnifier. And it can update the enabled state,
mEnabled, when it is called.

Bug: 163784304
Test: atest WindowMagnificationManagerTest,atest WindowMagnificationGestureHandlerTest
Change-Id: I62a5aeaf45a3df62ef995906f25129125b4e85d5