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
13 files changed