TetheringConfiguration: force to select upstream automatically on U+

Simply the upstream selection since U+ device for tethering refactor.

Changes:
1. Support automatic mode only on U+ device.

2. Add two tethering tests to make sure that the resource config can't
   control automatic mode anymore. Expect that always automatic mode
   on U+ device. Also limit some existing tethering tests for R, S,
   or T.
   Changes of TetheringTest:
   - testAutomaticUpstreamSelectionWithConfigDisabled (U+) [new]
   - testLegacyUpstreamSelection (T-)
   - testChooseDunUpstreamByLegacyMode (T-)
   - testChooseDunUpstreamByAutomaticModeWithConfigDisabled (U+) [new]

3. Add two tethering configuration tests to make sure that automatic
   mode is always enabled no mater what the resource config or flag
   are on U+ device. Also limit some existing tethering tests for R,
   S, or T.
   Changes of TetheringConfiguration:
   - testChooseUpstreamAutomatically (T-)
   - testChooseUpstreamAutomaticallyAfterT (U+) [new]
   - testChooseUpstreamAutomatically_FlagOverride (R)
   - testChooseUpstreamAutomatically_FlagOverrideOnSAndT (S, T)
     (Renamed from testChooseUpstreamAutomatically_FlagOverrideAfterR)
   - testChooseUpstreamAutomatically_FlagOverrideAfterT (U+) [new]

Bug: 257941865
Bug: 257408654
Test: atest TetheringTest
Change-Id: I7ace317a1529177a84557741dc07d732b7f8cf89
3 files changed