Allow touchpads to expose associated display

After landing ag/24209603, a touchpad test started failing; the
reason is that the device never exposed the associated display id,
even though it became associated to a viewport. This CL makes the
necessary changes so the associated display id is properly
propagated.

Bug: 293488187
Test: atest of the failing test
Change-Id: Ia1480012bd339de92abd284c6d00f7ac93505735