[connman] Share a single connection to D-Bus system bus.

[connman] Share a single connection to D-Bus system bus.

Previously each GDBusProxy to a Connman object had its own
system bus connection.  That was wasteful and unnecessary.
All Connman D-Bus proxies now share a single connection.  This
reduces resource utilization and improves performance in some
cases, e.g. the settings daemon WiFi connect/disconnect
implementation.

Change-Id: Idec33eeb90f8403170adc4f9eec5984bb98b5ce6
Signed-off-by: Ossama Othman <ossama.othman@intel.com>