L2SS-592: Avoid accessing stale values after OFF->STANDBY

Jan David Molrequested to merge
L2SS-592-tests-fail-on-attributes into master

L2SS-592: Tell Tango explictly when our attributes can be accessed, so that Tango knows that a client request after turning the device on needs to force a read immediately.

Closes L2SS-592

Edited by Jan David Mol

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