[java] Add TextUtils.isEmpty model

Summary:
Add a partial copy of TextUtils from Android source for commonly used TextUtils.isEmpty method.

Fixes #141
Closes https://github.com/facebook/infer/pull/143
Github Author: Deniz Türkoglu <deniz@spotify.com>

infer/models/java/src/android/text/TextUtils.java

@@ -0,0 +1,28 @@
+/*
+ * Copyright (C) 2006 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Partial copy of the file for static analysis purposes
+ */
+package android.text;
+
+public class TextUtils {
+    public static boolean isEmpty(CharSequence str) {
+        if (str == null || str.length() == 0) {
+            return true;
+        } else {
+            return false;
+        }
+    }
+}

infer/tests/codetoanalyze/java/infer/NullPointerExceptions.java

@@ -9,6 +9,7 @@ import android.content.ContentResolver;
 import android.content.Context;
 import android.database.Cursor;
 import android.database.sqlite.SQLiteDatabase;
+import android.text.TextUtils;
 
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
@@ -329,4 +330,11 @@ public class NullPointerExceptions {
       int n = s.length();
     }
   }
+
+  void nullableNonNullStringAfterTextUtilsIsEmptyCheckShouldNotCauseNPE(@Nullable String str) {
+    if(!TextUtils.isEmpty(str)) {
+      str.length();
+    }
+  }
+
 }