Refactor to use WindowManager#getCurrentWindowMetrics()

instead of using Display#getRealSize()

Since Display#getRealSize / getRealMetrics will be deprecated soon
in ag/13474172, and we also found an once issue b/180833541 that
related with it, so refactoring it.

Bug: 180833541
Test: atest WMShellUnitTests
Change-Id: I14220768b0c470ca3bdec410a107347000e9d511