Captive portals: reinspect http result after fallback probe

When DNS queries that precede the http and https probe take a long time
to complete, it is possible that the fallback probe fires earlier than
either the http and https probes.

This causes the detection logic to ignore the result of the http probe,
which may cause the system to miss captive portals advertising
themselves on the url used by the http probe but not on the url used by
the fallback probe.

This patch fixes that issue by forcing the detection to wait on the
result of the http probe if the fallback probe does not find a portal.

Bug: 38259299
Bug: 36532213
Test: manually tested with nearby captive portal networks
Change-Id: I07ea23a2b5f694c5ada9633169af98409efedff1