Stop using CaptivePortalData in the shim interfaces.

CaptivePortalDataShim exists to represent a CaptivePortalData
object in contexts where that object is not known to exist at
compile time.

However, the shim interfaces use CaptivePortalData objects
directly. This means the shims themselves cannot be compiled
unless CaptivePortalData exists at compile time.

This CL removes CaptivePortalData from the shim interface. This
will allow the API 29 shims can be compiled with sdk_version 29,
providing extra compile-time safety.

Test: atest NetworkStackCoverageTests NetworkStackTests NetworkStackNextTests
Change-Id: I1b3706ad39f839875a1761df55b70822bd21b9d1