Add lazy evaluation for getWindowInsets()

It is very common that apps call
windowManager.getCurrentWindowMetrics().getBounds();
windowManager.getMaximumWindowMetrics().getBounds();
without calling WindowMetrics#getWindowInsets().

Then the heavy operations IWindowManager#getWindowInsets
and InsetsState#calculateInsets are wasteful.

This change makes the calculation only be called if
WindowMetrics#getWindowInsets() is called.

Though it may make the result invalid if the interval between
creating WindowMetrics and calling getWindowInsets() is long,
the common usage should get the result immediately instead of
storing the WindowMetrics and get the outdated state later.

Bug: 151908239
Test: atest android.view.WindowMetricsTest

Change-Id: I28230adcdf004eed60a9fcd4481639f5c04b7c2c