Make availability to com.android.tethering explicit

This is done implicitly today via a baseline map in
build/soong/apex/apex.go

The hidl_interface also requires //apex_available:platform in its
apex_available. This lib gets installed in vendor, and
//apex_available:platform is required on all non-apex variants including
the vendor variants

Bug: 281077552
Test: m nothing
Change-Id: I6f46244f9122fb3e025af53f3a42ad441284a723