Provide a library interface to the virtual touchpad service.

And make VrWindowManager pay for^H^H^H^H^H^H^H^H use it. The library
interface is a step toward moving 2D embedding into VrCore because it
hides the binder connection that isn't directly accessible to VrCore.

As a matter of implementation, the touchpad-service client library
and the low-level evdev touchpad are derived from the same interface.

Bug: b/35990873
Test: VirtualTouchpad_test; integration TBD.
Change-Id: Ic922ff223ddd5a44f6fb4433a271f8341e93a0e7