audio policy: fix mute delay when setting output device

The duration of the temporary mute applied on each output
when switching device must be more than twice the actual delay
we wait for the mute to be effective at the output.
Otherwise some outputs with short latencies will be unmuted before the
device switch is actually executed.

This change makes sure that we take the maximum wait time into account
when requesting the delayed unmuting of each output.

Change-Id: Id92c201b47786082a44b6a5c1c8015273835a42e