Always get window insets from server

The previous implementation of computeWindowInsets is to
use last view from WM#addView, which may be invalid because
the last added view is removed and throws NPE when getting
property from the invalid view.
In this CL, we change the approach to always obtain window
insets from server.

fixes: 148789183
Test: atest FrameworksCoreTests:WindowMetricsTest
Test: atest WindowMetricsTests

Change-Id: Ic5512c31f86c597d99e13d29df654d3f527d6b55