a11y: Only show the cursor following mode setting with detected mouse

This makes sure the cursor following mode setting to be visible only
when a mouse device is detected.

Bug: b/398066000
Flag: com.android.settings.accessibility.enable_magnification_cursor_following_dialog
Test: com.android.settings.accessibility.ToggleScreenMagnificationPreferenceFragmentTest
Change-Id: I7dd27e9d8a841461900e28d8ae07d17f86a0b662