[Tether] Release flags related to Instant Tether

Flipping 3 flags all together, which should make it easier to roll back
if necessary.

Test: all SystemUI tests
Test: build
Flag: LEGACY SIGNAL_CALLBACK_DEPRECATION ENABLED
Flag: LEGACY WIFI_TRACKER_LIB_FOR_WIFI_ICON ENABLED
Flag: LEGACY INSTANT_TETHER ENABLED
// Instant tether bug
Bug: 265342892
// Flag bugs
Bug: 291315866
Bug: 292533677
Bug: 293585143

Change-Id: Ic2d5fa940bc865c0053cdd54a9ae1fd417992be2