Erroneous untrusted provider toast with LEAP fixed

I've also rearranged how we handle SSL errors. In first place, if the
user checked the "Trust completely" checkbox, leap_android tries to use
the CA cert pointed out by provider.json.
If that does not succeed, it uses an always-returning-true hostname
verifier (this was previously done before using CA cert).
Certificate message shown when appropiate.

Only if all downloading methods fail and the last one throws an
IOException, the certificate error message toast is shown.
If IOException, use current provider CA cert.

If the CA cert was correctly downloaded, we assume it can be used to
validate the string download.

If CA cert cannot validate that connection, then if the trust completely
checkbox was checked it will try with the other methods. If it was not
checked, a certificate error is shown (telling the user the
provider is not trusted).
Merge branch 'bug/incorrect-leap-untrusted-message' into develop