add preferences screen

first setting controls start on boot
rename services to better describe their function

* GpgAgentService only runs a local gpg-agent

* SharedDaemonsService runs daemons that are shared by multiple apps/uids
remove TODO.md, moved content to https://dev.guardianproject.info/issues/1235
add dirmngr to SharedDaemonsService so any app/UID can use the shared one