[Wi-Fi] Use getUriHelpString() result to judge if show the Openroaming help button.

The Openroaming help button should show only when the getHelpUriString() value is not null.

Bug: 146669261
Test: Adjust unit test cases:
      1. make RunSettingsRoboTests ROBOTEST_FILTER=WifiEntryPreferenceTest
      2. make RunSettingsRoboTests ROBOTEST_FILTER=WifiSettings2Test
Change-Id: I3d59162b782d5aca5251a42009eac95432d446d9