Move TetheringConfiguration out to its own file

Test: as follows
    - built (bullhead)
    - flashed
    - booted
    - runtest frameworks-net
    - vanilla tethering works
        - changing from no dun requirement to dun shows dun required
Bug: 32163131

Change-Id: I488449570dff057311b37335923ce6a199b363b7
2 files changed