The z-coordinate at which to place this app bar when it is above other content. This controls the size of the shadow below the app bar.
If this property is null, then
ThemeData.appBarTheme.elevation is used,
if that is also null, the default value is 4, the appropriate elevation
for app bars.
If forceElevated is false, the elevation is ignored when the app bar has no content underneath it. For example, if the app bar is pinned but no content is scrolled under it, or if it scrolls with the content, then no shadow is drawn, regardless of the value of elevation.
final double elevation