SAP: Fix PTS issues for disconnection from server

The SAP spec suggest to use a gracefull disconnect
when initiating a disconnect from a server, hence it is not a good
idea to expose a force disconnect option to the user without first
having tried a gracefull disconnect. Hence the PTS test case
implementation is in conflict with the intented use specified in
the SAP specification.

Without this fix a build with PTS_TEST=false will not work as
intented.

PTS_TEST replaced with system property bt.sap.pts. Set it to true
to enable the force disconnect button.

Change-Id: I28d0bb91b683c903842618a6f82b3bd49ec810e4