Return a copy of the read-only identity Matrix

An app trying to modify the returned matrix (despite what the
documentation says...) would crash when the matrix is identity.

Change-Id: I8aae06b712907085088fb9f0321352dd2c3462ac

core/java/android/widget/ImageView.java

@@ -562,13 +562,14 @@ public class ImageView extends View {
 
     /** Return the view's optional matrix. This is applied to the
         view's drawable when it is drawn. If there is not matrix,
-        this method will return null.
-        Do not change this matrix in place. If you want a different matrix
-        applied to the drawable, be sure to call setImageMatrix().
+        this method will return an identity matrix.
+        Do not change this matrix in place but make a copy.
+        If you want a different matrix applied to the drawable,
+        be sure to call setImageMatrix().
     */
     public Matrix getImageMatrix() {
         if (mDrawMatrix == null) {
-            return Matrix.IDENTITY_MATRIX;
+            return new Matrix(Matrix.IDENTITY_MATRIX);
         }
         return mDrawMatrix;
     }