Clean up WifiTetherPreferenceController

In previous code there are two main issues:
1. It listens to update from both WIFI_AP_STATE_CHANGED_ACTION and
ACTION_TETHER_STATE_CHANGED. It is unnecessary because they provides
same info(whether wifi hotspot is enabled, enabling...)

2. New API softApCallback already covers the
WIFI_AP_STATE_CHANGED_ACTION, so we don't need this broadcast anymore.

This cl fixes those two issues by cleaning up BroadcastReceiver and
update the tests.

Bug: 72702183
Test: RunSettingsRoboTests
Change-Id: I21c2818e0f0185172f34447a1716dc47ee065e23