Fix bugs regarding system app cannot write visible path

1. some system app by OEM can not write file to sdcard through visible
path("/storage/[UUID]") despite it has WRITE_MEDIA_STORAGE.

2. Google photo can't open and delete file which transferred from PC to
SDcard by MTP. (http://b/116332048)

Test: Check system app which has sdcard_rw permission writes file to sdcard("/storage/[UUID]").

Change-Id: Id38bd7c4753a38b6c61fff512821becfbcd9215a
Signed-off-by: Sangho Yoon <sangho.yoon@lge.com>

data/etc/platform.xml

@@ -62,6 +62,7 @@
 
     <permission name="android.permission.WRITE_MEDIA_STORAGE" >
         <group gid="media_rw" />
+        <group gid="sdcard_rw" />
     </permission>
 
     <permission name="android.permission.ACCESS_MTP" >

media/java/android/mtp/MtpStorage.java

@@ -37,7 +37,7 @@ public class MtpStorage {
 
     public MtpStorage(StorageVolume volume, int storageId) {
         mStorageId = storageId;
-        mPath = volume.getInternalPath();
+        mPath = volume.getPath();
         mDescription = volume.getDescription(null);
         mRemovable = volume.isRemovable();
         mMaxFileSize = volume.getMaxFileSize();

services/core/java/com/android/server/pm/PackageManagerService.java

@@ -24,6 +24,7 @@
 import static android.Manifest.permission.REQUEST_DELETE_PACKAGES;
 import static android.Manifest.permission.SET_HARMFUL_APP_WARNINGS;
 import static android.Manifest.permission.WRITE_EXTERNAL_STORAGE;
+import static android.Manifest.permission.WRITE_MEDIA_STORAGE;
 import static android.content.Intent.ACTION_MAIN;
 import static android.content.Intent.CATEGORY_DEFAULT;
 import static android.content.Intent.CATEGORY_HOME;
@@ -21691,6 +21692,9 @@ public int getMountMode(int uid, String packageName) {
                 if (Process.isIsolated(uid)) {
                     return Zygote.MOUNT_EXTERNAL_NONE;
                 }
+                if (checkUidPermission(WRITE_MEDIA_STORAGE, uid) == PERMISSION_GRANTED) {
+                    return Zygote.MOUNT_EXTERNAL_DEFAULT;
+                }
                 if (checkUidPermission(READ_EXTERNAL_STORAGE, uid) == PERMISSION_DENIED) {
                     return Zygote.MOUNT_EXTERNAL_DEFAULT;
                 }