update(slider): improve disabled attribute validation

Improved the `disabled` attribute validation by only observing the attribute instead of using a getter function - as same as done in the `autocomplete`

  Closes angular#7310

src/components/slider/slider.spec.js

@@ -366,6 +366,8 @@ describe('md-slider', function() {
 
     pageScope.$digest();
 
+    pageScope.$digest();
+
     // Doesn't add active class on pressdown when disabled
     wrapper.triggerHandler({
       type: '$md.pressdown',
@@ -388,6 +390,8 @@ describe('md-slider', function() {
 
     pageScope.$digest();
 
+    pageScope.$digest();
+
     // Check for disabled state by triggering the pressdown handler and asserting that
     // the slider is not active.
     wrapper.triggerHandler({