fix(#AlwaysOnMagnification): mis-activate fullscreen magnifier when display size changed

If the AlwaysOn feature is enabled, when onDisplaySizeChanged() is triggered (such as screen rotation), the magnifier would set scale to 1.0x even when not activated, and thus the magnifier becomes activated. Therefore, we add state check at the beginning to prevent the issue.

Bug: 271438152
Test: manually
      atest FullScreenMagnificationControllerTest
Change-Id: Ic489904812a3207a48d7fdb93421e300ccd1b81f