Add getDescendants to Settings Slice Provider

getDescendants allows another app to collect a list
of all valid Uris for Settings Slices. Important for search,
assistant or launcher apps.

This means searching our database for all valid keys and building
a list of Uris for each of those keys.

Fixes: 77808328
Test: robotests
Change-Id: I3ae27e4661a7dcaab50b091ae2730013118af8a2

src/com/android/settings/slices/SliceBuilderUtils.java

@@ -130,7 +130,7 @@ public class SliceBuilderUtils {
         // Example: "/action/wifi" -> [{}, "action", "wifi"]
         //          "/action/longer/path" -> [{}, "action", "longer/path"]
         if (split.length != 3) {
-            throw new IllegalArgumentException("Uri (" + uri + ") has incomplete path: " + path);
+            return null;
         }
 
         final boolean isInline = TextUtils.equals(SettingsSlicesContract.PATH_SETTING_ACTION,