Fix icon color in dark theme of gallery (#7544)

Previously, the icon in the tooltip demo was black on black background
in the dark theme. Now it is white on black background in the dark theme
and black on white background in the light theme.

fixes #7018

examples/flutter_gallery/lib/demo/tooltip_demo.dart

@@ -33,7 +33,7 @@ class TooltipDemo extends StatelessWidget {
                     child: new Icon(
                       Icons.call,
                       size: 18.0,
-                      color: theme.primaryColor
+                      color: theme.iconTheme.color
                     )
                   ),
                   new Text(' icon.', style: theme.textTheme.subhead)
@@ -43,7 +43,7 @@ class TooltipDemo extends StatelessWidget {
                 child: new IconButton(
                   size: 48.0,
                   icon: new Icon(Icons.call),
-                  color: theme.primaryColor,
+                  color: theme.iconTheme.color,
                   tooltip: 'Place a phone call',
                   onPressed: () {
                     Scaffold.of(context).showSnackBar(new SnackBar(