Track input state when transferring touch focus.

Copies the input state to the destination window and sends synthesic
cancelation events to the source window.

Change-Id: Ia75820b0d756ed5d6cd22dce7830251ac85141ed
diff --git a/include/ui/InputDispatcher.h b/include/ui/InputDispatcher.h
index 2c22cad..63185d3 100644
--- a/include/ui/InputDispatcher.h
+++ b/include/ui/InputDispatcher.h
@@ -757,6 +757,9 @@
         // Clears the current state.
         void clear();
 
+        // Copies pointer-related parts of the input state to another instance.
+        void copyPointerStateTo(InputState& other) const;
+
     private:
         struct KeyMemento {
             int32_t deviceId;