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.h b/services/vr/virtual_touchpad/VirtualTouchpadService.h
index 05a2a50..e2426e3 100644
--- a/services/vr/virtual_touchpad/VirtualTouchpadService.h
+++ b/services/vr/virtual_touchpad/VirtualTouchpadService.h
@@ -8,6 +8,9 @@
 namespace android {
 namespace dvr {
 
+// VirtualTouchpadService implements the service side of
+// the Binder interface defined in VirtualTouchpadService.aidl.
+//
 class VirtualTouchpadService : public BnVirtualTouchpadService {
  public:
   VirtualTouchpadService(VirtualTouchpad& touchpad)
@@ -22,6 +25,7 @@
  protected:
   // Implements IVirtualTouchpadService.
   ::android::binder::Status touch(float x, float y, float pressure) override;
+  ::android::binder::Status buttonState(int buttons) override;
 
  private:
   VirtualTouchpad& touchpad_;