Change FlakyTest from android.test.FlakyTest to our version

BUG=

Review URL: https://codereview.chromium.org/1903533002

Cr-Commit-Position: refs/heads/master@{#389897}

content/public/android/javatests/src/org/chromium/content/browser/NavigationTest.java

@@ -4,10 +4,10 @@
 
 package org.chromium.content.browser;
 
-import android.test.FlakyTest;
 import android.test.suitebuilder.annotation.MediumTest;
 
 import org.chromium.base.test.util.Feature;
+import org.chromium.base.test.util.FlakyTest;
 import org.chromium.base.test.util.UrlUtils;
 import org.chromium.content.browser.test.util.TestCallbackHelperContainer;
 import org.chromium.content.browser.test.util.TestCallbackHelperContainer.OnEvaluateJavaScriptResultHelper;