Bump the project minimum SDK API

[fdocr]

Is your feature request related to a problem? Please describe.

Some initial attempts bumping the project minimum SDK API haven't been successful, but it's still an important thing we want to do in order to give a better experience to users since we do rely on some calls that are not supported in the current minimum (API 18).

Describe the solution you'd like

At the moment targeting support for API 21 and above feels like the right balance between reaching as many devices as possible while making sure we are installing in devices that will get a good experience. An example of this is that WebView's evaluateJavascript is only supported in API 19+ and we currently support a minimum 18 (which means they don't get Push Notifications and other features that rely on this).

Additional context

Contributions in the form of PRs are very welcome. Work on this issue may overlap with #87, just in case anyone is working on this it may be good to keep an eye on that one and see if conflicts may be avoided beforehand.