ASOC: qdsp6:Add pause/resume support for AAC encoder

-  If session is stopped, AUDIO_START should
   trigger session to be started again. This way
   pause/resume becomes stop/start for AAC encoder

Change-Id: Id44a5342bb90b049d9c617ede60bf41a5289c581
Signed-off-by: Sharad Sangle <assangle@codeaurora.org>