mdMedia improvements

[gkalpak]

This PR contains some improvements to mdMedia, split-up in 3 commits as follows:

1. fix(mdUtil): remove/delete cacheFactory keys when clearing/destroying cache:
   mdUtil's custom cacheFactory factory, adds/removes keys when putting/removing a single key, but leaves keys untouched when calling removeAll() or destroy(). As a result, the following problems arise:

   • Keys of destroyed caches continue to take up memory.
   • The keys() method return incorrect results after calling removeAll() (i.e. it continues to return the old keys, although they are not present in the cache any more).

   This commit fixes these issues, by overriding the removeAll and
   destroy methods as well.

2. fix(mdMedia): avoid unnecessary digest and make changes apply quicker:
   Previously, the updateAll() method relied on $timeout for scheduling a digest. This made the possible changes apply after the next rendering and could possibly lead to extra digests.
   This commit, replaces the call to $timeout with a call to $evalAsync(), which avoids an extra digest if one is already in progress and also applies the changes before the next rendering.

3. test(mdMedia): add more tests and move .spec file to correct location:
   In commit 6397040 mdMedia was moved out of the sidenav component and into core. The .spec file remained in the original folder though.*
   This commit moves the 'media.spec.jsfile to the correct location and adds more tests to more thoroughly covermdMedia`'s functionality.

   _{* Since this commit, the file has been moved ot the right location in cfd48f8.}

[googlebot]

CLAs look good, thanks!

[gkalpak]

/ping @ThomasBurleson (or anyone)

[ThomasBurleson]

@gkalpak - think this is dangerous. Perhaps:

cache._destroy = cache.destroy;
    cache.destroy = function() {
      keys = {};
      return cache._destroy();
    };

is safer and allows subsequent calls to cache.put() to work properly.

Even though semantically the cache.put( ) should not be called after cache.destroy( )

[gkalpak]

Subsequent calls to put() will not work correctly anyway, because the original put() will fail after having called destroy(). (put() is not supposed to be called after destroy().)

var cache = $mdUtil.cacheFactory('test');
cache.destroy();
cache.put('key', 'value');   // Throws an error regardless of the value of `keys`

So, having keys = null helps GC (instead of having to keep a reference to an empty object around). But, it's impact should be minimal (even more so considering destroy() is rarely used currently).