Declare "searchable" attribute for preferences.

Now we can easily mark a preference nonIndexable in xml instead of
adding key into searchIndexProvider.

Bug: 112608186
Test: robotests
Change-Id: I0ff16d44bb7b6ad148d3d35f09ca0da0163f73f4

tests/robotests/src/com/android/settings/security/EncryptionAndCredentialTest.java

@@ -73,7 +73,7 @@ public class EncryptionAndCredentialTest {
         final List<String> expectedKeys = new ArrayList<>();
         for (SearchIndexableResource res : index) {
             expectedKeys.addAll(((BaseSearchIndexProvider) SEARCH_INDEX_DATA_PROVIDER)
-                    .getNonIndexableKeysFromXml(mContext, res.xmlResId));
+                    .getNonIndexableKeysFromXml(mContext, res.xmlResId, true /* suppressAll */));
         }
         final List<String> keys = SEARCH_INDEX_DATA_PROVIDER.getNonIndexableKeys(mContext);