The tethering unit tests were in FrameworksNetTests, but were moved out
so were no longer run in presubmit. Now put them to postsubmit first.
Should be moved to presubmit after a while.
Bug: 148998835
Test: atest --test-mapping frameworks/base/packages/Tethering:postsubmit
Change-Id: I9ca36cba7bfde8409a06dbeb7166337c55b39691
