Support video attachments through GalleryMediaChooser

GalleryMediaChooser supports video types and more image types also. And
video thumbnails are overlaid with the play button image to distinguish
between images and videos.

Note:
 1. EXTRA_ALLOW_MULTIPLE is not specified for ACTION_GET_CONTENT.
 2. Files, methods and variables' name are still including "image".
 3. Content descriptions are not updated.

Test: Manual

Change-Id: I961928f150e4ae8ee80a1fba2f20c37fb5426669
Signed-off-by: Taesu Lee <taesu82.lee@samsung.com>

src/com/android/messaging/ui/mediapicker/MediaPicker.java

@@ -159,7 +159,7 @@ public class MediaPicker extends Fragment implements DraftMessageSubscriptionDat
     @VisibleForTesting
     final Binding<MediaPickerData> mBinding = BindingBase.createBinding(this);
 
-    /** Handles picking image from the document picker */
+    /** Handles picking a media from the document picker. */
     private DocumentImagePicker mDocumentImagePicker;
 
     /** Provides subscription-related data to access per-subscription configurations. */