speech: Update recognition-service resource

Adding an attribute to the recognition-service resource which will
limit the maximum number of concurrent sessions ongoing with the same
recognition implementation. The default value of attribute is 1
(no concurrency), maintaining the back-compatibility.

Each recognition service implementation can override this value for
itself to the extent they find suitable.

Bug: 260713947
Test: n/a
Change-Id: I84d4856c8bb99a73f2b542c207429c5dcc7e1e91