Make members final in TetheringService

1. Move isTetheringSupport logic from TetheringService to Tethering.
2. Small readability improvement in TetheringTest. Also change
config_tether_upstream_automatic from false to true in TetheringTest.
So TetheringTests would default run automatic select upstream flow
instead of selecting by legacy perferred network type list.

Bug: 153609486
Test: atest TetheringTest
Change-Id: I5a82a6347f62d3a7031db5c56e8e0c8530dafd8f
Merged-In: I5a82a6347f62d3a7031db5c56e8e0c8530dafd8f