Support many at-spi clients
Commit allows many clients to enable the at-spi2 bus by
setting the org.a11y.Status.IsEnabled property to true.
Commit assumption:
clients have to maintain an open connection to the session dbus
after setting the org.a11y.Status.IsEnabled property,
last closed connection will disable the at-spi2 bus.
Commit requires: https://review.tizen.org/gerrit/#/c/117565/
https://review.tizen.org/gerrit/#/c/129497/
https://review.tizen.org/gerrit/#/c/129494/
Change-Id: Id47067b448e5765d76472797c8c0e37fdbef0be0