Bug 1065279 - [settings] refactor Hotspot panel with AMD pattern

The test relies on the value store under `tethering.wifi.security.password` and the value is not initialized before executing the test, which means changes to `MockNavigatorSettings` may affect the test here.
This test is more like a black-box test. You should initialize the value prior to creating an instance of `HotspotSettings` because `_init` is called during the creation.