Fix incorrect regex in WebView useragent test.

The test was updated to make the useragent more flexible but the new
regex incorrectly expected a semicolon inbetween the model and build ID,
which isn't supposed to be there.

Fix the regular expression to require the correct spacing/semicolons;
this requires a more complex regex, so comment the regex and also add
tests for the test itself with known-good/bad forms of useragent.

Fixes: 125402818
Change-Id: Iea63668835a399e7feffc9af8c3aef6488ab1fbe