Use a sysprop to enable `TouchpadInputMapper`

The new touchpad behaviour can now be toggled using the `setprop`
command followed by a userspace reboot instead of having to change the
const and rebuild.

Bug: 251196347
Test: set the flag with setprop, check the gestures library logs appear
Test: atest inputflinger_tests
Change-Id: Ie4db00219e915fd059647d7541ef84e27e75bd86