Allow multiple contact lists to be selected and synced per account

[AutoImport-oliv3r]

Right now, you are only allowed to sync 1 contact list per davdroid account. While less common, it is possible to actually have multiple contact accounts available to you.

Removing the restriction to only select one, but allow multiple would be more user friendly.

[rfc2822]

Android provides only one address book while it provides multiple calendars. Thus, only one address book can be synced. Creating multiple accounts is an option, but in this case, the user should do so.

The restriction is not by DAVdroid but by Android – there can be only one address book per account.

Using groups would be a possible solution but there are other bugs preventing this (see #48).