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As the media framework does not directly support marker and periodic
callbacks, we use a retriggerable one-shot timer at application level
to emulate this.  This has the advantage of being faster, but is also
less accurate than if it were in the framework.

Details:
 - Get position is now synchronous and (mostly) lock-free
 - Fix regression in SL_PLAYEVENT_HEADATMARKER and SL_PLAYEVENTHEADATNEWPOS
 - Fix SMP races in GetPlayState, GetCallbackEventsMask, GetMarkerPosition,
   GetPositionUpdatePeriod
 - AudioSfDecoder was not updating mPositionMsec when getPositionUsec failed
 - Updating attributes is relatively expensive, so only do it when a significant change
    in SetMarkerPosition, ClearMarkerPosition, SetPositionUpdatePeriod
 - Error on unknown message type in onMessageReceived
 - Delete an obsolete FIXME
 - "Clearing" a marker is now equivalent to disabling the marker event,
   and the default marker position is cleared.
 - use Android time units and types (int32_t, ANDROID_UNKNOWN_TIME, etc.) where appropriate
   instead of SLmillisecond and SL_TIME_UNKNOWN

Change-Id: Ib78bafa20d883b8d927364769663837389b6ea1d