Move thread_util to libnetdutils

This change comprises:

[1] Move thread_util.h to libnetdutils to ease the cleanup of the
    include path system/netd/server for libnetd_resolv.

[2] Rename thread_util.h to ThreadUtil.h.

[3] Add the test to ThreadUtilTest.

Bug: 128662167
Test: system/netd/tests/runtests.sh passed
Change-Id: Ia8e9b196fbe41a4bd1edc7592badaacf57165988
diff --git a/libnetdutils/ThreadUtilTest.cpp b/libnetdutils/ThreadUtilTest.cpp
new file mode 100644
index 0000000..2fe63b7
--- /dev/null
+++ b/libnetdutils/ThreadUtilTest.cpp
@@ -0,0 +1,65 @@
+/*
+ * Copyright (C) 2019 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.
+ */
+
+#include <gtest/gtest.h>
+
+#include "netdutils/ThreadUtil.h"
+
+namespace android {
+namespace netdutils {
+
+namespace {
+
+class NoopRun {
+  public:
+    NoopRun() { instanceNum++; }
+    ~NoopRun() { instanceNum--; }
+
+    void run() {}
+
+    static bool waitForAllReleased(int timeoutMs) {
+        constexpr int intervalMs = 20;
+        int limit = timeoutMs / intervalMs;
+        for (int i = 1; i < limit; i++) {
+            if (instanceNum == 0) {
+                return true;
+            }
+            usleep(intervalMs * 1000);
+        }
+        return false;
+    }
+
+    // To track how many instances are alive.
+    static std::atomic<int> instanceNum;
+};
+
+std::atomic<int> NoopRun::instanceNum;
+
+}  // namespace
+
+TEST(ThreadUtilTest, objectReleased) {
+    NoopRun::instanceNum = 0;
+    NoopRun* obj = new NoopRun();
+    EXPECT_EQ(1, NoopRun::instanceNum);
+    threadLaunch(obj);
+
+    // Wait for the object released along with the thread exited.
+    EXPECT_TRUE(NoopRun::waitForAllReleased(1000));
+    EXPECT_EQ(0, NoopRun::instanceNum);
+}
+
+}  // namespace netdutils
+}  // namespace android