daemon: fix PropertiesChanged signal emission

This fixes a few problems of PropertiesChanged signal emission.
First, it should be sent only when the properties have changed.
Second, invalidated_properties should be set when the properties have
been reset, not on error.  Finally, the sender should be
"org.freedesktop.IBus" rather than "org.freedesktop.DBus".

BUG=
R=takao.fujiwara1@gmail.com

Review URL: https://codereview.appspot.com/11465045