Remove NetworkMonitor dependency on ICaptivePortal

ICaptivePortal is used in the framework and cannot be used as a
dependency in NetworkMonitor, as the framework class takes precedence
when linking.
Also fix NetworkMonitorTest that was not verifying the right
startCaptivePortalApp call.

(cherry-pick of aosp/904953 with minor conflict in Android.bp)

Test: atest FrameworksNetTests NetworkStackTests
Bug: 124033493
Change-Id: Ib6a89e54312628662b130fbeec18d11e139f09fa