[audio] Move sending volume intent to under lock

Modification of this intent is not thread safe. Since dispatching of the
intent should be in-order wrt the actual set change, move all of the
intent generation and enqueuing code underneath the VSS lock.

Test: compiles
Bug: 374206971
Flag: EXEMPT bugfix
Change-Id: Id023eeed49db2e473e92e3e0d1b328054eacf2c7