Fix the indicator of magnification around the mirror window

The indicator of magnification is around the mirror window
because it's unmagnifiable. This phenomenon happens when switching
the magnification mode. To have better transition experience, we
exclude the A11y magnfication overlay when calculating the
magnification region.

Bug: 153852242
Test: manual test: show mirror window when the border is visible.
Change-Id: Ie95a04012dc31ef844a7f5eea87c2f2e253b6533