Make Hearing devices preference always visible

Previously we hide the preference when Bluetooth is off since it's only used for showing connected hearing aids and acting as a shortcut to device detail page.

However, the preference is now a new page of multiple controls, we probably shouldn't hide the preference the Bluetooth is off. Those controls should be accessible even if the Bluetooth is off.

Bug: 284092084
Test: make RunSettingsRoboTests ROBOTEST_FILTER=HearingAidHelperTest
Change-Id: I647a187d73bd673c8ac8ab366579ba92c22ce1dc