Forgot to reset the matrix of offscreen canvas

Follow up to I5bc980c4dc41bc304be3c190934c1f7c55c9d244

bug: 8967766
Change-Id: I37c2e8e70ff4966ca5e4f29e5cac8349d8fe73b6

java/src/com/android/inputmethod/keyboard/internal/GestureTrailsPreview.java

@@ -110,6 +110,7 @@ public final class GestureTrailsPreview extends AbstractDrawingPreview {
 
     private void freeOffscreenBuffer() {
         mOffscreenCanvas.setBitmap(null);
+        mOffscreenCanvas.setMatrix(null);
         if (mOffscreenBuffer != null) {
             mOffscreenBuffer.recycle();
             mOffscreenBuffer = null;