[AccountSettings] Start add account in new task

Start add account activities in a new task separate from settings
so that users can reach it from recents later, even if they navigate
to a different settings screen (via launcher) like selecting Wi-Fi.

Bug: 22351974
Change-Id: Icd3e20fa8f2426276c52cd11cbc916c56ca86ee3

src/com/android/settings/accounts/AddAccountSettings.java

@@ -99,6 +99,7 @@ public class AddAccountSettings extends Activity {
                             Utils.hasMultipleUsers(AddAccountSettings.this));
                     addAccountOptions.putParcelable(EXTRA_USER, mUserHandle);
                     intent.putExtras(addAccountOptions);
+                    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
                     startActivityForResultAsUser(intent, ADD_ACCOUNT_REQUEST, mUserHandle);
                 } else {
                     setResult(RESULT_OK);