PTS: Add a Secure Only mode.

Use Case:
Certification test cases require BLE to be in secure only
mode and reject legacy pairing requests.

Test Case: TP/SEC/SEM/BV-28-C

Failure:
Currently the host doesn't have a method of putting host into
a mode where it can only perform secure pairing.
This prevents execution of certification test cases
requiring Secure Only mode.

Fix:
Made changes to read the value of a tag "PTS_SecurePairOnly"
at runtime from the file "bt_stack.conf".

Bug: 27852645
Change-Id: I32cd6412621efeed2cb31c760a031762d9f369f9