Separating the UI logic for SettingsPanel and ModeSwitchButton

The existence of magnification settings panel is controlled in
system UI. In MagnificationController, we would only like to guarantee
that the settings panel is removed when the situation that could show
settings panel is not satisfied.

Bug: 279377146
Test: Manually - attach video to the bug
Change-Id: I1db69a48fab4779fbec11080966f1ba08932b49f