不需要自己实现 onAccessibilityEvent 方法，直接使用 UiAutomator 的 UiDevice.performActionAndWait() 方法就可以了处理 AccessibilityEvent 事件。
http://android.xsoftlab.net/reference/android/support/test/uiautomator/UiDevice.html
核心是实现一个检查 Toast 的 EventCondition 类。可以参考 android.support.test.uiautomator.Until.scrollFinished 实现。
可以通过命令行工具查看具体的 event 信息。

adb shell uiautomator events > d:/events.log 

由于 EventCondition 的两个抽象方法是包访问权限，因此必须新建一个 android.support.test.uiautomator 同名的包。
具体代码如下：

package android.support.test.uiautomator;
import android.view.accessibility.AccessibilityEvent;
import java.util.ArrayList;
import java.util.List;
public class MyUntil {
    //检查给定Toast文案是否展示
    public static EventCondition<Boolean> toastShown(final CharSequence toastText) {
        return new EventCondition<Boolean>() {
            private int mMask = AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED;
            private boolean mIsClassMatch = false;
            private static final String FILTER_CLASS_NAME = "android.widget.Toast$TN";
            private boolean mResult = false;
            private List<CharSequence> mText;

            @Override
            Boolean apply(AccessibilityEvent event) {
                mMask &= ~event.getEventType();
                mIsClassMatch = FILTER_CLASS_NAME.equals(event.getClassName());
                mText = event.getText();
                mResult = mMask == 0 && mIsClassMatch && mText.contains(toastText);
                return mResult;
            }

            @Override
            Boolean getResult() {
                return mResult;
            }
        };
    }
    //获取Toast的文案
    public static EventCondition<String> getToastText() {
        return new EventCondition<String>() {
            private int mMask = AccessibilityEvent.TYPE_NOTIFICATION_STATE_CHANGED;
            private boolean mIsClassMatch = false;
            private static final String FILTER_CLASS_NAME = "android.widget.Toast$TN";
            private List<CharSequence> mText = new ArrayList<>();

            @Override
            Boolean apply(AccessibilityEvent event) {
                mMask &= ~event.getEventType();
                mIsClassMatch = FILTER_CLASS_NAME.equals(event.getClassName());
                mText = new ArrayList<>(event.getText());//必须要复制getText否则，在getResult中获取不到结果。
                return mMask == 0 && mIsClassMatch;
            }

            @Override
            String getResult() {
                String result = null;
                if (mText.size() > 0) {
                    result = mText.get(0).toString();
                }
                return result;
            }
        };
    }
}

使用方法:

import android.support.test.uiautomator.MyUntil;//导入类

Runnable action = new Runnable() {
    @Override
    public void run() {

    }
};
EventCondition<Boolean> condition = MyUntil.toastShown("已经同步到最新");
assertTrue(mDevice.performActionAndWait(action,condition,3000));
//EventCondition<Boolean> condition = MyUntil.getToastText();
//assertThat(mDevice.performActionAndWait(action,condition,3000),is("已经同步到最新"));