PTS: Allow disabling connection updates

Use Case: Certification test case requiring LE secure pairing
with PTS.

Failure:
LE pairing test cases follow the host behavior where
connection parameters are switched to fast parameters
for SMP and service discovery and switched back to the
default values after SMP/LE discovery is complete.
PTS disconnects to these series of connection updates.

Fix:
This change disables connection updates based on run time
flag "PTS_DisableConnUpdates" fetched from bt_stack.conf

Bug: 27852645
Change-Id: I1da213fd143ccfcb0ac82c7049a94094d2716ff1