fix(input): make md-maxlength validation happen on initialization with interpolated value (#11338)

Closes #11329


## PR Checklist
Please check that your PR fulfills the following requirements:
- [x] The commit message follows [our guidelines](https://github.com/angular/material/blob/master/.github/CONTRIBUTING.md#-commit-message-format)
- [x] Tests for the changes have been added or this is not a bug fix / enhancement
- [x] Docs have been added, updated, or were not required

## PR Type
What kind of change does this PR introduce?
```
[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:
```

## What is the current behavior?

Using md-maxlength="MAX" on an input field that initializes to some value of length > "MAX", the md-maxlength validation does not happen, and the input field is considered valid.

Issue Number: #11329 

## What is the new behavior?

The validation now happens with specified value and error is detected

## Does this PR introduce a breaking change?
```
[ ] Yes
[x] No
```
<!-- If this PR contains a breaking change, please describe the impact and migration path for existing applications below. -->
<!-- Note that breaking changes are highly unlikely to get merged to master unless the validation is clear and the use case is critical. -->

## Other information

Author: feloy

src/components/input/input.js

@@ -642,6 +642,10 @@ function mdMaxlengthDirective($animate, $mdUtil) {
     var ngTrim = angular.isDefined(attr.ngTrim) ? $mdUtil.parseAttributeBoolean(attr.ngTrim) : true;
     var isPasswordInput = attr.type === 'password';
 
+    scope.$watch(attr.mdMaxlength, function(value) {
+      maxlength = value;
+    });
+
     ngModelCtrl.$validators['md-maxlength'] = function(modelValue, viewValue) {
       if (!angular.isNumber(maxlength) || maxlength < 0) {
         return true;
@@ -685,7 +689,6 @@ function mdMaxlengthDirective($animate, $mdUtil) {
       });
 
       scope.$watch(attr.mdMaxlength, function(value) {
-        maxlength = value;
         if (angular.isNumber(value) && value > 0) {
           if (!charCountEl.parent().length) {
             $animate.enter(charCountEl, errorsSpacer);

src/components/input/input.spec.js

@@ -291,6 +291,42 @@ describe('md-input-container directive', function() {
       expect(getCharCounter(el).text()).toBe('5 / 5');
     });
 
+    it('should error with an interpolated value and incorrect initial value', function() {
+      var el = $compile(
+        '<form name="form">' +
+        '  <md-input-container>' +
+        '    <input md-maxlength="mymax" ng-model="foo" name="foo">' +
+        '  </md-input-container>' +
+        '</form>')(pageScope);
+
+        pageScope.$apply('mymax = 8');
+        pageScope.$apply('foo = "ABCDEFGHIJ"');
+
+      // Flush any pending $mdUtil.nextTick calls
+      $timeout.flush();
+
+      expect(pageScope.form.foo.$error['md-maxlength']).toBe(true);
+      expect(getCharCounter(el).text()).toBe('10 / 8');
+    });
+
+    it('should work with an interpolated value and correct initial value', function() {
+      var el = $compile(
+        '<form name="form">' +
+        '  <md-input-container>' +
+        '    <input md-maxlength="mymax" ng-model="foo" name="foo">' +
+        '  </md-input-container>' +
+        '</form>')(pageScope);
+
+      pageScope.$apply('mymax = 5');
+      pageScope.$apply('foo = "abcde"');
+
+      // Flush any pending $mdUtil.nextTick calls
+      $timeout.flush();
+
+      expect(pageScope.form.foo.$error['md-maxlength']).toBeFalsy();
+      expect(getCharCounter(el).text()).toBe('5 / 5');
+    });
+
     it('should work with a constant', function() {
       var el = $compile(
         '<form name="form">' +