Added support for custom Anki Server URL (SYNC_BASE).

Hi,

This is a small PR that adds the ability for users to specify a custom SYNC_BASE url in the preferences. The default is the https://ankiweb.net/ URL.

Would anyone have a suggestion as how to replicate the functionality I added in AndroidApp.getApplicationPrefs() so it may not be necessary?

This also does not verify if the given URL ends with a slash. It is currently left to the user to format this url properly.
Please use the constant from Consts.java instead of hard-coding it here.