Add startCaptivePortalApp to system API

The API is already used by settings and should be usable by setup
wizards. It is the only way for a caller outside of the system_server to
trigger the captive portal application.

The API is already CTS tested in android.net.cts.CaptivePortalTest.

Fixes: 182871577
Test: atest CtsNetTestCases:android.net.cts.CaptivePortalTest
Change-Id: Ie8d9a546b54524ba837715baa94a07d1f993d8d3