[A11y Magnification Keyboard Focus] Only show follow keyboard setting when keyboard is attached

* Add check to only show keyboard focus following toggle setting if a
  keyboard is attached.
* Add test to check that setting is not shown if no keyboard is
  attached.
* Make peripheral detection utils methods public for use in
  accessibility settings package.

Flag: android.view.accessibility.request_rectangle_with_source
Bug: 414831974
Bug: 405120787
Bug: 391877896
Test: Manual testing, ToggleScreenMagnificationPreferenceFragmentTest
Change-Id: I38e75a124db85a4ef2021078e4ca5a6e918b4f4c