Use system browser for login on iOS and Android

Currently, when logging in to Dropbox, we use a web view on iOS and Android. This is needed in order to avoid the OS putting the app into the background (which cuts the network connection and hence prevents us from receiving the callback we wait for from the oAuth login).

However, this has security implications and even prevents the app from being officially approved for productive Dropbox usage.

We shall check how we can properly use the system browser (like other apps do) and switch to this approach for both OSes.