fix(select/datepicker): Fix dropdown icon colors. (#10226)

Change the dropdown icon colors for Select and Datepicker to be
darker by default.

*Old Styles*

 - Default: foreground-3 (light grey)
 - Hover: foreground-2 (darker grey)

*New Styles*

 - Default: foreground-2 (darker grey)
 - Disabled: foreground-3 (light grey; select only; datepicker
   hides the icon when disabled)

Note that the hover effect appeared not to work and since it used
the proper default color, it has been removed.

src/components/datepicker/datePicker-theme.scss

@@ -33,10 +33,6 @@
 
   .md-datepicker-triangle-button {
     .md-datepicker-expand-triangle {
-      border-top-color: '{{foreground-3}}';
-    }
-
-    &:hover .md-datepicker-expand-triangle {
       border-top-color: '{{foreground-2}}';
     }
   }

src/components/select/select-theme.scss

@@ -45,6 +45,7 @@ md-select.md-THEME_NAME-theme {
     background-image: linear-gradient(to right, '{{foreground-3}}' 0%, '{{foreground-3}}' 33%, transparent 0%);
     background-image: -ms-linear-gradient(left, transparent 0%, '{{foreground-3}}' 100%);
   }
+
   .md-select-value {
     border-bottom-color: '{{foreground-4}}';
 
@@ -56,18 +57,22 @@ md-select.md-THEME_NAME-theme {
       color: '{{warn-A700}}'
     }
   }
+
   &.md-no-underline .md-select-value {
     border-bottom-color: transparent !important;
   }
+
   &.ng-invalid.ng-touched {
     .md-select-value {
       color: '{{warn-A700}}' !important;
       border-bottom-color: '{{warn-A700}}' !important;
     }
+
     &.md-no-underline .md-select-value {
       border-bottom-color: transparent !important;
     }
   }
+
   &:not([disabled]):focus {
     .md-select-value {
       border-bottom-color: '{{primary-color}}';
@@ -76,31 +81,36 @@ md-select.md-THEME_NAME-theme {
         color: '{{ foreground-1 }}';
       }
     }
+
     &.md-no-underline .md-select-value {
       border-bottom-color: transparent !important;
     }
+
     &.md-accent .md-select-value {
       border-bottom-color: '{{accent-color}}';
     }
+
     &.md-warn .md-select-value {
       border-bottom-color: '{{warn-color}}';
     }
   }
+
   &[disabled] {
     .md-select-value {
       color: '{{foreground-3}}';
+
       &.md-select-placeholder {
         color: '{{foreground-3}}';
       }
     }
+
+    .md-select-icon {
+      color: '{{foreground-3}}';
+    }
   }
 
   .md-select-icon {
-    color: '{{foreground-3}}';
-
-    &:hover {
-      color: '{{foreground-2}}';
-    }
+    color: '{{foreground-2}}';
   }
 }