Tidy up Geary.AccountInformation creation.

Pass service provider to ctor so we can make the service provider
property immutable. Fill in service label in ctor so it's always set to
something useful. Move creating orphans from Engine to AccountManager
since it exists only to assign an internal id for new accounts, so it
should handled by the account manager anyway.

test/engine/app/app-conversation-monitor-test.vala

@@ -30,6 +30,7 @@ class Geary.App.ConversationMonitorTest : TestCase {
     public override void set_up() {
         this.account_info = new AccountInformation(
             "account_01",
+            ServiceProvider.OTHER,
             new MockServiceInformation(),
             new MockServiceInformation()
         );