Don't allow to install older builds

In some cases, installing an update older than the one currently
installed requires a data wipe, so don't allow it. However, allow
to verify old updates if not verified yet (e.g. when imported).

src/org/lineageos/updater/misc/Utils.java

@@ -85,7 +85,9 @@ public class Utils {
     }
 
     public static boolean canInstall(Update update) {
-        return update.getVersion().equalsIgnoreCase(SystemProperties.get(Constants.PROP_BUILD_VERSION));
+        return update.getTimestamp() > SystemProperties.getLong(Constants.PROP_BUILD_DATE, 0) &&
+                update.getVersion().equalsIgnoreCase(
+                        SystemProperties.get(Constants.PROP_BUILD_VERSION));
     }
 
     public static List<UpdateDownload> parseJson(File file, boolean compatibleOnly)