Try to fix stale mDisplaySize

WM#getMaximumWindowMetrics#getBounds report max window bounds with
the Configuration in Resources.
However, EdgeBackGestureHandler#onConfigurationChanged counld not
guarantee that WM#getMaximumWindowMetrics is updated because
the configuration updates are not on the same Context, and also
it Resources#getConfiguration.
This CL applies the reported Configuration from #onConfigurationChanged
callback to mDisplaySize directly. In this way, mDisplaySize is
up-to-date with the latest configuration.

Bug: 235711424
Test: manual - rotate the device dozens of times and check if
        mDisplaySize is updated.
Change-Id: Iafeb1c580bb0c7b3680a5c32616754c00666768c