Set preference visible to false when there is no keyboard.

1. Physical keyboard settings should only show up if at least one PK is connected.
2. Update tests.
3. Fix NullPointerException.

Bug: 269983475
Bug: 270109384
Bug: 271357910
Test: atest and manual
Change-Id: If7798587fb386dbf669fc249cab304d91a26879b

src/com/android/settings/inputmethod/PhysicalKeyboardFragment.java

@@ -160,6 +160,10 @@ public final class PhysicalKeyboardFragment extends SettingsPreferenceFragment
         final Context context = getContext();
         ThreadUtils.postOnBackgroundThread(() -> {
             final List<HardKeyboardDeviceInfo> newHardKeyboards = getHardKeyboards(context);
+            if (newHardKeyboards.isEmpty()) {
+                getActivity().finish();
+                return;
+            }
             ThreadUtils.postOnMainThread(() -> updateHardKeyboards(newHardKeyboards));
         });
     }