Support highlightable Settings homepage menu for 2-pane

1. Add a preference group adapter to perform highlighting
2. Add a class for mapping highlighable menu keys and preference keys
3. Add an API to determine if the screen is in split mode

Bug: 199017944
Test: manual, build
Change-Id: I8e3fe5fb96480a31ee0f3b3afb6ad78999d3d2bc

src/com/android/settings/SettingsPreferenceFragment.java

@@ -119,8 +119,7 @@ public abstract class SettingsPreferenceFragment extends InstrumentedPreferenceF
 
     @VisibleForTesting
     public HighlightablePreferenceGroupAdapter mAdapter;
-    @VisibleForTesting
-    public boolean mPreferenceHighlighted = false;
+    private boolean mPreferenceHighlighted = false;
 
     @Override
     public void onCreate(Bundle icicle) {