Revise virtual touchpad interface.

- Explicit start and stop, outside of which the evdev devices
  don't exist.
- Permission test (not compiled by default pending build & SELinux
  support for temporarily retaining a second copy of the service
  for vr_wm).
- Enforce a single user of the touchpad.
- Support 'dumpsys'.

Bug: 36051900
Test: log inspection
Change-Id: I038ed2632d5adf50a3565a981031691d5dc5f7cd
diff --git a/services/vr/virtual_touchpad/VirtualTouchpadEvdev.h b/services/vr/virtual_touchpad/VirtualTouchpadEvdev.h
index ec8006b..b158cec 100644
--- a/services/vr/virtual_touchpad/VirtualTouchpadEvdev.h
+++ b/services/vr/virtual_touchpad/VirtualTouchpadEvdev.h
@@ -18,15 +18,17 @@
   static sp<VirtualTouchpad> Create();
 
   // VirtualTouchpad implementation:
+  status_t Attach() override;
+  status_t Detach() override;
   status_t Touch(int touchpad, float x, float y, float pressure) override;
   status_t ButtonState(int touchpad, int buttons) override;
+  void dumpInternal(String8& result) override;
 
  protected:
   VirtualTouchpadEvdev() {}
   ~VirtualTouchpadEvdev() override {}
-  status_t Initialize();
 
-  // Must be called only between construction and Initialize().
+  // Must be called only between construction and Attach().
   inline void SetEvdevInjectorForTesting(EvdevInjector* injector) {
     injector_ = injector;
   }