audiod: Avoid delay in status transition from standby to on.

audiod: Avoid delay in status transition from standby to on.

In close_receiver() we invalidate the current audio format value
to prevent the receiver from re-starting too quickly. See commit
2d61a8005c74 (audiod: Invalidate current audio format on close)
from seven years ago for details.

This invalidation happens in particular when the receiver is closed
because audiod was told to enter standby mode. If audiod is switched
back on shortly thereafter, no receiver can't be started because we
forgot the current audio format and must wait up to one minute until
the next status item dump arrives from the server which re-initializes
the current audio format value.

Fix this flaw by only invalidating the current audio format when
audiod operates in "on" mode.

Fixes: 2d61a8005c74