Update app widget size when resized

Instead of relying on the passed-in size, we calculate the size of the
view after layout and update the widget with that size. This ensures the
widget is always up-to-date with the right size.

Bug: 368056517
Test: manually by expanding and shrinking widgets
Flag: com.android.systemui.communal_widget_resizing
Change-Id: Ief764b582df5e5784d63444d33a2171f6dd5e4be