Riseup's EE certificate expired (EE = End-Entity, the commercial
certificate signed by the issuer), and we weren't able to fetch
provider.json because the pin wasn't valid.

This problem needs to be avoided in the future, using
HKPK (https://tools.ietf.org/html/rfc7469), which Micah's implementing
on Riseup.

Switching from build types from flavors enables us to run tests against
production apk, not just debug. I didn't detect this pinning problem
because tests were run only against the debug apk, which trusted
preseeded providers by default (thus bypassing pinning issue).

Next step: beautify.

I've separated the user session management to it, and encapsulated
ProviderAPICommand into its own class.

Putting the fragment statically in dashboard.xml isn't working, Android
complains about it being duplicated, so I'm going to add it dynamically.

A provider can be asked if it's configured or not, so that we don't find
more provider = null issues.

I've created a DefaultedURL which links to https://example.net, so that
we can initialize an URL. I use composition instead of inheritance,
since URL class is final.

Using AndroidPinning library from Moxie, I make sure the provider.json
file Bitmask downloads is fetched from a pinned https connection, so
that the api certificate fingerprint is the good one.

String locale = Locale.getDefault().getLanguage() + Locale.getDefault().getCountry();