Support for devices which don't have Google Play Services

[codeka]

Currently, WW depends on Google Play Services for the sign-in flow and for ads. If we want to put WW on more app stores than just Google Play, we need to be able to support devices which don't have Google Play Services installed.

Ads shouldn't be too hard, that just means finding a different provider.

Sign-in flow is a bit more tricky. Signing in with a Google account doesn't seem to work (from my testing at least) without going the GPS, at least on lollipop and up. That means we'll need to support user-name and passwords.