Assert add/removeCallbacks are from main thread in DevicePostureControllerImpl

There have been bugs associated to concurrent modifications of mListeners, but we're not able to reproduce them. With this cl, there will be a more obvious error message in future bugs, if there will be any.

Bug: 219510239
Test: Manually tried to trigger the assert, failing.
Change-Id: I9dd5e0fd45227b133b17f5a6680b35c89b183ce8