Use String in CaptivePortalData underlying types

The venue friendly name comes from the network, and only plain strings
should be used in practice. writeCharSequence is also not usable in
CaptivePortalData, as it must build using module_api.

Use a String as the underlying type in CaptivePortalData, keeping
CharSequence in the API for consistency with UI-related elements.

This brings CaptivePortalData in line with downstream branches.

Bug: 183174863
Test: atest FrameworksNetTests
Merged-In: Ifa3ea9aae0cabf5757791c4ae13e6f551759ed97
Change-Id: Ic662d3b546d52f825f9c3a24c5e0d4ba03818ab0