Improve handling of GATT timeout

If a dynamic channel is used, then on its closure, we should let the ATT
channel timeout. Similarly, if an ATT client exists and then is closed,
a timeout should occur.

*HOWEVER*, if only an ATT server exists, then we should not timeout.
This is to avoid breaking backwards compatibility.

Bug: 251954527
Test: atest pts-bot
Change-Id: I86cf1eeb358ce940cada326b6afa1cf3b02479f3