time: only consider packet threads

time: only consider packet threads

In offline mode, a timestamp is kept per thread, and the lowest
timestamp of the active threads is used. This was also considering the
non-packet threads, which could lead to the used timestamp being further
behind that needed. This would happen at the start of the program, as
the non-packet threads were set up the same way as the packet threads.

This patch both no longer sets up the timestamp for non-packet threads
as well as not considering non-packet threads during timestamp
retrieval.

Fixes: 6f560144c1b9 ("time: improve offline time handling")
Bug: #7034.