flag guarding getWindowInsets()

Reading insets was needed to properly centre the lock icon.
The insets were only included when calculating the mid-point when the lockscreen.enable_landscape flag is enabled. However, the insets where read outside of flag-guarding.
Now, the insets are only read if the flag is on.

Bug: 296571001
Bug: 293252410
Bug: 300763027

Test: LockIconViewControllerTest
Change-Id: I915642c7d6afc52b1f00d0874bd0bbd215feefa1