gpg-agent: Hook up the TTL cache option

 * So that when the gsettings gpg-cache-method is 'idle' or 'timeout'
   we use gpg-cache-ttl to control how long the passphrase is cached
   for.
 * This is a regression from 3.3.x

https://bugzilla.gnome.org/show_bug.cgi?id=681081