fix(material-experimental/mdc-button): extended fab touch target not covering entire button (angular#24322)

Fixes that the extended FAB touch target was square which meant that it didn't cover the entire button.

Author: crisbeto

src/material-experimental/mdc-button/fab.scss

@@ -89,3 +89,9 @@
     );
   }
 }
+
+// All FABs are square except the extended ones so we
+// need to set the touch target back to full-width.
+.mat-mdc-extended-fab .mat-mdc-button-touch-target {
+  width: 100%;
+}