CaptivePortalLoginActivity correctly gets UserAgent

Change I33cbcdab9809c712d998da6d925bdd4e3093f8ff added to the intent
passed from NetworkMonitor to the CaptivePortalLoginActivity an extra
argument to set the UserAgent header of probes done by the
CaptivePortalLoginActivity.

However that extra String argument is not correctly retrieved from the
Intent and instead a null pointer is returned by getParcelableExtra().
Consequently the UserAgent header field is never set.

This patch fixes this bug and replace getParcelableExtra() with
getStringExtra().

It also adds logging of the request header and response header for every
network probes sent by CaptivePortalLoginActivity.

Bug: 35937367, 32369183
Test: manually tested
Change-Id: If5c463023474ffb0401f3cda5875ccd9fc6f79f9