Make tethering APIs unsupported synchronously when disallowed

To backward compatible existing behavior, make tethering APIs
unsupported synchronously when tethering is disallowed. There
are two APIs register/unregisterTetheringEventCallback still
supported even tethering is disallowed.
This could avoid some existing tests flaky. The test assume
startTethering would fail right away after tethering restricted
apply.

Bug: 184996041
Bug: 239500515
Test: atest TetheringTests
Change-Id: I41ae1d61647c9baf69bcb246965e8d9b4a89b497
3 files changed