Add synthetic back button to virtual touchpad/stylus.

Also add a bit more error checking and documentation.

Test: added to VirtualTouchpad_test.cpp
Bug: 34673438
Change-Id: I3851a2ad79c5338cdd1db0c7d460aecfff082cc3
diff --git a/services/vr/virtual_touchpad/VirtualTouchpadService.cpp b/services/vr/virtual_touchpad/VirtualTouchpadService.cpp
index e5ead0e..5e3321f 100644
--- a/services/vr/virtual_touchpad/VirtualTouchpadService.cpp
+++ b/services/vr/virtual_touchpad/VirtualTouchpadService.cpp
@@ -13,11 +13,16 @@
 }
 
 binder::Status VirtualTouchpadService::touch(float x, float y, float pressure) {
-  // Permissions check added and removed here :^)
   const int error = touchpad_.Touch(x, y, pressure);
   return error ? binder::Status::fromServiceSpecificError(error)
                : binder::Status::ok();
 }
 
+binder::Status VirtualTouchpadService::buttonState(int buttons) {
+  const int error = touchpad_.ButtonState(buttons);
+  return error ? binder::Status::fromServiceSpecificError(error)
+               : binder::Status::ok();
+}
+
 }  // namespace dvr
 }  // namespace android