Remove Android dependency from fastStripHtml function

This just makes it easier to use it in tests without any mocks.

WordPressUtils/src/main/java/org/wordpress/android/util/HtmlUtils.java

@@ -37,7 +37,7 @@ public static String stripHtml(final String text) {
      * @return String without HTML
      */
     public static String fastStripHtml(String str) {
-        if (TextUtils.isEmpty(str)) {
+        if (str == null || str.isEmpty()) {
             return str;
         }