Allocate/free the SDP connection timers only during stack startup/shutdown

This avoids freeing the sdp_conn_timer within the alarm callback itself.

Bug: 67110137
Test: Manual
Change-Id: I775b4b532cd42cf207258c53c6052a167a124627
Merged-In: I775b4b532cd42cf207258c53c6052a167a124627
(cherry picked from commit ef6a4a0c9d9220a7d909863349d7a0c0b967d54c)