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
Merged-In: Iebd307b5d73a7b4679baea2a1c12c3baba6a7447
Merged-In: I851d8c41d42dccb03f813e549439f44c83f7d5ac
(clean cherry-pick of pi-dev I70acfd9213bf620cbb79ad999b8ad03472e8d43f)

Change-Id: I3b3bcbbd380f8cbd0b663556d55276f43e30b3e5