Working AndroidTestOrchestrator example

AndroidTestOrchestrator example would be nice. The JavaDoc instructions are not working for me.

```
*

Whereas previously you might have called {@code am instrument -w
* com.example.app/android.support.test.runner.AndroidJUnitRunner} you would now execute {@code
* 'CLASSPATH=$(pm path com.google.android.apps.common.testing.services) app_process /
* android.support.test.services.shellexecutor.ShellMain am instrument -w -e targetInstrumentation
* com.example.app/android.support.test.runner.AndroidJUnitRunner
* android.support.test.orchestrator/android.support.test.orchestrator.AndroidTestOrchestrator'}
```

I always get an error. The links to download the relevant apks are also missing.

首先检查 AndroidTestOrchestrator JavaDoc 命令和现有的 testing-samples 示例。重现报告的 instrumentation 错误，添加一个可运行的示例以及相关 APK 的链接，并验证文档中的命令是否有效。

由索引模型根据 Issue 内容生成。