[bt][tools] Fix bug in bt-le-central connection

Fixed regression caused by the FIDL event stream handler function
which didn't propagate the future returned by one of the match cases.
This caused the connect_peripheral future to not be polled and drop
its gatt.Client handle.

Test: "bt-le-central scan -c" connects, lists GATT services, and starts
the GATT REPL.

Change-Id: I49893aea74fe68b5783574bae3d577e120a18463