Add C interface for virtual touchpad client.

For use by the dvr api.

Changes VrWindowManager::ShellView to exercise this change,
and because it will be used when that code relocates to VrCore.

Bug: 36050790
Test: use in vr_wm.
Change-Id: I0fa67d19f94939ae41ef0ea2338f92f6147a4111
diff --git a/services/vr/virtual_touchpad/Android.bp b/services/vr/virtual_touchpad/Android.bp
index ad999b7..c8bc884 100644
--- a/services/vr/virtual_touchpad/Android.bp
+++ b/services/vr/virtual_touchpad/Android.bp
@@ -94,6 +94,7 @@
 
 client_src = [
     "VirtualTouchpadClient.cpp",
+    "DvrVirtualTouchpadClient.cpp",
     "aidl/android/dvr/VirtualTouchpadService.aidl",
 ]