[Thread] fix Thread unit test failures

It looks like SettingsUnitTests is not in presubmit and the existing Thread settings unit tests are failing in postsubmit.

This commit fixes two small issues:
1. The default Thread enabled state in FakeThreadNetworkController and ThreadNetworkToggleController is not consistent
2. The executor in ThreadNetworkToggleControllerTest is reusing the main thread which could result in flaky as the callbacks are not guaranteed to be invoked before the API returns

Bug: 339767488
Bug: 339145155
Test: atest SettingsUnitTests:com.android.settings.connecteddevice.threadnetwork.ThreadNetworkToggleControllerTest
Change-Id: Id8b453cf6d7bef698d08dbf106e48250369d0832