[Signin][Android] Reset shared prefs in OAuth2TokenServiceIntegrationTest

This CL adds SigninHelper.resetSharedPrefs call to
OAuth2TokenServiceIntegrationTest.tearDown method.
OAuth2TokenServiceIntegrationTest has been causing many test failures in
different tests because of the state it has been leaving in shared
preferences.

Bug: 908542
Change-Id: I10df9d0ab1d268c7da3352ca87ae07659bfa55d2
Reviewed-on: https://chromium-review.googlesource.com/c/1368147Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614777}

chrome/android/javatests/src/org/chromium/chrome/browser/signin/OAuth2TokenServiceIntegrationTest.java

@@ -87,6 +87,7 @@ public class OAuth2TokenServiceIntegrationTest {
             mChromeSigninController.setSignedInAccountName(null);
             mOAuth2TokenService.validateAccounts(false);
         });
+        SigninHelper.resetSharedPrefs();
     }
 
     private void mapAccountNamesToIds() {