Renamed certificate handling methods.

I still let one method to bypass hostname verifier, otherwise dev.bm
will not work.

I've reduced downloading functions to 3: download without CA, download
with commercial CA and download with provider CA.

I only download CA cert once, when the user selects the corresponding
provider. Each time a user switch provider, the cert is downloaded
(along with other json files).
AlertDialog for failed downloads.

The dialog shows only an "OK" button to dismiss it, and the reason why
the download failed.

Right now, this dialog appears if a download is not successful. If it's
too intrusive, I can remove some cases. Let's try how it feels.