Rename VirtualTouchpadService to IVirtualTouchpadService

aidl compiler no longer force prepend I when the interface name does not
start with I.

Bug: 129549755
Test: m libvirtualtouchpadclient
Merged-In: I973b5138d671fb842160d2795caa1c8eb977f642
Change-Id: I973b5138d671fb842160d2795caa1c8eb977f642
(cherry picked from commit e0067979)