Support many at-spi clients

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