Use CaptivePortalProbeSpec method in portal app

This only changes behavior if the EXTRA_CAPTIVE_PORTAL_PROBE_SPEC extra
is passed, which only happens if extra probe specs have been defined in
settings.

Bug: b/79499239
Test: manual: on 2 different portals, works w/ and w/o the setting
Change-Id: I70acfd9213bf620cbb79ad999b8ad03472e8d43f