Magnification window should not block bottom navigation bar

To prevent a dragging gesture from misdetecting as a swipe up gesture
when moving the magnification window from the bottom navigation bar,
the magnification window should not overlapped the gesture-detection
region of the bottom navigation bar.
So we have to constrain the position of the magnification window if the
navigation bar mode is NAV_BAR_MODE_2BUTTON or NAV_BAR_MODE_GESTURAL.

Bug: 151394480
Test: atest com.android.systemui.accessibility
Change-Id: If0e3b8bcdcde8ce218e6f5087922783be6ab80c3