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
58df155b9d