Support multiple virtual touchpads.

The service now creates two uinput devices, named
"vr virtual touchpad 0" for VirtualTouchpad::PRIMARY
and "vr virtual touchpad 1" for VirtualTouchpad::VIRTUAL.

Bug: b/35992608
Test: expanded VirtualTouchpad_test unit test
Change-Id: I8749d559a56cfa9c8ec9a039ff7ad44d16f81915
4 files changed