UiAutomator findObject is throwing IllegalStateException: UiAutomation not connected!

Getting below Exception for every consecutive execution

```
java.lang.IllegalStateException: UiAutomation not connected!
at android.app.UiAutomation.throwIfNotConnectedLocked(UiAutomation.java:971)
at android.app.UiAutomation.waitForIdle(UiAutomation.java:577)
at android.support.test.uiautomator.UiAutomatorBridge.waitForIdle(UiAutomatorBridge.java:112)
at android.support.test.uiautomator.UiDevice.waitForIdle(UiDevice.java:639)
at android.support.test.uiautomator.UiDevice.waitForIdle(UiDevice.java:629)
at android.support.test.uiautomator.UiDevice.getWindowRoots(UiDevice.java:1074)
at android.support.test.uiautomator.UiDevice.findObject(UiDevice.java:164)
```

observed throwing above exception immediately after executing below statement in logcat

`W WindowManager: Attempted to remove non-existing token: android.os.Binder@2728fc2`

**Tested on API Level:** 19, 21, 23 emulators, issue also replicating in real device
**Uiautomator Version:** com.android.support.test.uiautomator:uiautomator-v18:2.1.2

Please let me know if you need any other information.

先从 UiDevice.findObject 堆栈跟踪和之前的 WindowManager logcat 警告入手，然后使用 UiAutomator 2.1.2 在列出的 API 级别上重现该故障。当连续执行不再产生 UiAutomation not connected 异常，并且已确定触发条件时，即表示完成。

由索引模型根据 Issue 内容生成。