activity: Use parent's app id if available
This is usually more reliable of e.g. a dialog's app-id. This has several TODOs in the code (hence draft), I mostly wanted to do a quick check if this is works as intended.
/cc @eliasrudberg as this came out of #958 (closed)
Closes: #959 (closed) Signed-off-by: Guido Günther agx@sigxcpu.org