floss: listen for thread_event_cb to exit main thread

Instead of listening to adapter state cbs to exit sigterm,
listen to thread event cbs since that signals that the stack has
been cleaned up. Also serialize stack cleanup.

Bug: 293236498
Tag: #floss
Test: mma -j32
Test: turn off adapter and verify stack cleanup via logs
Change-Id: I5c9e320691c4927c9f7d9a517e41132dac83ea4c