Messaging doesn't allow sharing of its own files

-Previously, Messaging allowed sharing of any file it had permission
 to reach. This meant that bad apps could share a link to
 file:///data/data/com.android.messaging/databases/bugle_db
 and Messaging would happily send all this sensitive information to
 the target. Worse, a bad app could share a softlink to this file,
 where the symlink was picture.jpg with the image/jpg type.
-Now, when sanitizing attachments, we make sure any filepaths don't
 lead to any Bugle-specific directories.
-getApplicationInfo().dataDir is a symlink to
 /data/data/com.android.messaging, and appears to be the
 only directory where we store personal data.
-Most apps share as contentUris, including Messaging, so Messaging
 can still share to itself.

Change-Id: Ic464bc1f099029a030793c478aaf88b957d8bad1
Fixes:28076752

src/com/android/messaging/util/FileUtil.java

@@ -16,7 +16,10 @@
 
 package com.android.messaging.util;
 
+import android.content.ContentResolver;
 import android.content.Context;
+import android.net.Uri;
+import android.text.TextUtils;
 import android.webkit.MimeTypeMap;
 
 import com.android.messaging.Factory;
@@ -116,6 +119,20 @@ public class FileUtil {
         }
     }
 
+    private static boolean isFileUri(final Uri uri) {
+        return TextUtils.equals(uri.getScheme(), ContentResolver.SCHEME_FILE);
+    }
+
+    // Checks if the file is in /data/data/com.android.messaging
+    // The other app folders are either symlinks to this, or hold non-private data like binaries.
+    public static boolean isInPrivateDir(Context context, Uri uri) {
+        if (!isFileUri(uri)) {
+            return false;
+        }
+        final File file = new File(uri.getPath());
+        return FileUtil.isSameOrSubDirectory(new File(context.getApplicationInfo().dataDir), file);
+    }
+
     /**
      * Checks, whether the child directory is the same as, or a sub-directory of the base
      * directory.