Index database before getting Uri Descendants

onGetSliceDescendants would return empty results if
the call was made before the database was indexed.
This CL checks the index before building the list.

Change-Id: I2e0f88893138a048dbd529d465d68fa4b1a3dc12
Fixes: 78196823
Test: robotests

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

@@ -89,6 +89,7 @@ public class SlicesDatabaseAccessor {
      * @return a list of keys in the Slices database matching on {@param isPlatformSlice}.
      */
     public List<String> getSliceKeys(boolean isPlatformSlice) {
+        verifyIndexing();
         final String whereClause;
 
         if (isPlatformSlice) {