Support bimanual gesture input

Change-Id: Id86dc93fb7115ab4a6d05b3c05652df35ad08c12

java/src/com/android/inputmethod/keyboard/KeyboardView.java

@@ -992,7 +992,7 @@ public class KeyboardView extends View implements PointerTracker.DrawingProxy {
     }
 
     @Override
-    public void showGestureTrail(PointerTracker tracker) {
+    public void showGesturePreviewTrail(PointerTracker tracker) {
         locatePreviewPlacerView();
         mPreviewPlacerView.invalidatePointer(tracker);
     }

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

@@ -130,8 +130,12 @@ public class GestureStroke {
     }
 
     private void appendBatchPoints(final InputPointers out, final int size) {
+        final int length = size - mLastIncrementalBatchSize;
+        if (length <= 0) {
+            return;
+        }
         out.append(mPointerId, mEventTimes, mXCoordinates, mYCoordinates,
-                mLastIncrementalBatchSize, size - mLastIncrementalBatchSize);
+                mLastIncrementalBatchSize, length);
         mLastIncrementalBatchSize = size;
     }