Convert all database notifiers to use DatabaseObserver.

Lots of red in this diff to celebrate the release of Red (Taylor's Version).

app/src/main/java/org/thoughtcrime/securesms/MediaPreviewActivity.java

@@ -568,26 +568,11 @@ public final class MediaPreviewActivity extends PassphraseRequiredActivity
       if (item == 0) {
         viewPagerListener.onPageSelected(0);
       }
-
-      cursor.registerContentObserver(new ContentObserver(new Handler(getMainLooper())) {
-        @Override
-        public void onChange(boolean selfChange) {
-          onMediaChange();
-        }
-      });
     } else {
       mediaNotAvailable();
     }
   }
 
-  private void onMediaChange() {
-    MediaItemAdapter adapter = (MediaItemAdapter) mediaPager.getAdapter();
-
-    if (adapter != null) {
-      adapter.checkMedia(mediaPager.getCurrentItem());
-    }
-  }
-
   @Override
   public void onLoaderReset(@NonNull Loader<Pair<Cursor, Integer>> loader) {
 
@@ -701,11 +686,6 @@ public final class MediaPreviewActivity extends PassphraseRequiredActivity
     public boolean hasFragmentFor(int position) {
       return mediaPreviewFragment != null;
     }
-
-    @Override
-    public void checkMedia(int currentItem) {
-
-    }
   }
 
   private static void anchorMarginsToBottomInsets(@NonNull View viewToAnchor) {
@@ -824,14 +804,6 @@ public final class MediaPreviewActivity extends PassphraseRequiredActivity
       return mediaFragments.containsKey(position);
     }
 
-    @Override
-    public void checkMedia(int position) {
-      MediaPreviewFragment fragment = mediaFragments.get(position);
-      if (fragment != null) {
-        fragment.checkMediaStillAvailable();
-      }
-    }
-
     private int getCursorPosition(int position) {
       if (leftIsRecent) return position;
       else              return cursor.getCount() - 1 - position;
@@ -870,6 +842,5 @@ public final class MediaPreviewActivity extends PassphraseRequiredActivity
     void pause(int position);
     @Nullable View getPlaybackControls(int position);
     boolean hasFragmentFor(int position);
-    void checkMedia(int currentItem);
   }
 }