Fix `symbol not found` error for hidden apis used by MTS

This fails to build on git_master when
depending on `sdk_version:module-current` and
`libs:["framework-tethering-pre-jarjar"].

AIUI, some builds use sdk prebuilts that have compiled out
some of the hidden apis. And thus fails with symbol not found.

So we depend on the impl version of frame-tethering so we always
have access to these hidden apis for MTS.

Test: atestNetHttpCoverageTests
Change-Id: Id68a9051b9f8566f9ca42142669150a6db8b0925