adb: switch version back to 40.

The version bump we did wasn't actually needed, since adbd doesn't
check for either feature flag we added. Revert the change and clarify
the comment suggesting the version bump when adding features.

Test: ./test_adb.py
Test: ./test_device.py
Change-Id: I92e7f392bcb36b3bf9a236f2d31ba5133de8d72a