The two sides of a ASHA pair rarely if ever start together and the
sequence number was always a bit off due to the stateful nature of
reset_buffer and ASHA needing the sequence number to be matched to
the other side.
Simplify this by setting the sequence number for ASHA just before
flushing.
In ASHA, we might need to sync sequence numbers between
left and right side media sink. If the sequence number
is added in start encode, it becomes difficult to set
the sequence number again when a call to reset_buffer
in media sink might have happened already.
This effectively reverts commit ab5f81b9a.
