Make CaptivePortalProbeSpec and CaptivePortalProbeResult as a library

These two classes were added to @SystemApi because they are used
both by NetworkMonitor and CaptivePortalLogin. However it turns
out they are not needed in the framework, so having them as a
library sounds better.

Change-Id: Iadf77ec5952b6da8812dc6d006a39bd4e93d2bd9
Fix: 129433264
Test: atest NetworkStackTests FrameworksNetTests