Mark a hidden MotionEvent method to be visible for tests

This allows CTS coverage for MotionEvent.isButtonPressed.

Bug: 31441257
Change-Id: I885b596f9f6f94c1cb48ff61c8002a28b4dd5ead

api/test-current.txt

@@ -42103,6 +42103,7 @@ package android.view {
     method public final void recycle();
     method public final void setAction(int);
     method public final void setActionButton(int);
+    method public final void setButtonState(int);
     method public final void setEdgeFlags(int);
     method public final void setLocation(float, float);
     method public final void setSource(int);

core/java/android/view/MotionEvent.java

@@ -2334,6 +2334,7 @@ public final class MotionEvent extends InputEvent implements Parcelable {
      * @see #getButtonState()
      * @hide
      */
+    @TestApi
     public final void setButtonState(int buttonState) {
         nativeSetButtonState(mNativePtr, buttonState);
     }