CaptivePortalData: use CharSequence in VenueFriendlyName API

Following up on feedback from API council, change the String type
to CharSequence in the get and set Venue friendly name API.

Bug: 179163405
Test: atest ConnectivityServiceTest CtsNetTestCasesLatestSdk:CaptivePortalDataTest
Test: atest NetworkNotificationManagerTest NetworkMonitorTest
Change-Id: Ia63974cd2ff5975bde410eb93731d4b1def36d19