Cleanup execution flow through SpecialSearch::execute()

Cleanup execution flow through SpecialSearch::execute()

Not a big change, but makes it so there is only one path into
SpecialSearch::showResults(). Makes things a little easier to follow.
Also moves the code for checking if full text search is disabled into
execute(), makes sense to check before even trying to make a search
engine. Also moves some setup code out of execute and into the setupPage
function

Bug: T150393
Change-Id: Ib527fc3a3c39eb2e56985e5d1e4905fc4562353c