Set the security mode to secure connections only mode

Use case: Test the secure connections only mode PTS test cases.

Test Steps:
1. Call the API BTM_SetSecureConnectionsOnly from BT-IF layer to
   put the DUT is under secure connections only mode to execute
   the PTS test case TP/SEC/SEM/BV-11-C.
2. Run the required steps for PTS test case.

Failure: Failed to enter into secure connections only mode.

Root Cause: The function BTM_SetSecureConnectionsOnly doesn't set
btm layer security mode variable to only mode and all further checks
are based on that variable.

Fix: Fixed it by setting the security mode variable properly
in the API function.

Bug: 21896912
Change-Id: I21d37e7d24c443c2cbcd72d6f62f5e31fa7cf107