Rename tethering util folder and its package name

Move tethering util files from android.net.util into
com.android.networkstack.tethering.util. The goal is move all of
tethering internal files into its own namespace
com.android.networkstack.tethering.util.

Bug: 205088391
Test: atest TetheringTests
      atest CtsTetheringTest
      atest TetheringPrivilegedTests

Change-Id: I6559fb4f873b3cad5b210b10e49df1b6c6914a70
23 files changed