Define menu keys for highlighting the menu entry on 2-pane UI

- Define an attribute for highlightable menu keys
- List the keys for Settings menu entries
- Support parsing the attribute

Bug: 199017944
Test: robotest
Change-Id: I5759bff591c79d498fffb8b2c2b57f2cb66c4421

tests/robotests/res/xml-mcc999/location_settings.xml

@@ -24,6 +24,7 @@
         android:title="title"
         android:icon="@drawable/ic_android"
         android:summary="summary"
+        settings:highlightableMenuKey="menu_key"
         settings:controller="com.android.settings.slices.FakePreferenceController"
         settings:keywords="a, b, c"
         settings:unavailableSliceSubtitle="subtitleOfUnavailableSlice"/>