Custom control panel

A long time ago I set up Prey with a custom control panel (CP) and modified the android client to point to my CP. This was working, but required manually changing the config file for each release, which caused my Prey for Android client to be quite outdated.

I added preferences that, if not empty, override the ones in the config file. Therefore, if the app gets updated, I do not have to manually build it from source (if these changes are accepted) to get the latest features. Since most users don't need this, and simply do not want to be bothered with too many settings, I hid these settings behind the version info, just like Google does with the developer tools in Android.

Note that these preferences are only available when an account has already been linked to the device. This means that when changing the control panel url, one has to be sure these settings are correct and that the device is already registered with the control panel specified in the 'config' file.