Bluetooth-OPP: Race condition issue fix while receiving files

During batch of files reciving from remote, Obex server is blocked
and unblocked before receiving each file.

In some race condition if unblock calls first and later blocked
the server, then receive will fail and server session will enter in
an infinite loop. This will cause successive receive failure.
So moved up server block flag and will set when DUT receives first
PUT request from remote.

Change-Id: Iae6eacbc2645bc06c655875aef72a8ef8b2cb76f