leaudio: Fix possible crash on receiving ISO data

This patch cleans flags for sending and receiving audio. Now enum AudioState
is used to indicated READY_TO_START and STARTED state.

READY_TO_START is used to audio driver requested Audio Resume and that
it has been accepted by Bluetooth.

STARTED is usedn when CISes and codec are ready and audio driver has
been notified that stream can start.

Bug: 150670922
Tag: #feature
Sponsor: jpawlowski@
test: manual

Change-Id: I8fcf2b2c15ff2267871cc8a9413c4968072ccb87