audio policy: fix delayed command insertion

When inserting a delayed command in the audio policy service
command thread queue, later similar commands are filtered out.
In case of volume commands and delayed unmuting, this can cause
a problem where a new volume command can cause the timing of a
delayed unmuting command to not be honored.

This change makes that a volume command gets the time stamp of
any delayed command it replaces.

Bug 6720482.

Change-Id: I216f85ac0ea46e4a046d3483e9e9b7ffe6c34a08