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