audio: Supress redundant effect config events in short duration

Distortion sound persists for the whole song when re-enabling effect in
LPA playback mode till switch to next song. And pop noise is quite obvious
when enable/disable effects in LPA.

As the design of LPA, EffectsThreadEntry() loop is used to handle effect
config changes through applying effects with new config on dsp pcm buffers.
However, it can't handle too many config change events in a short time,
and effect with new configuration will be applied on part of dsp buffers.

Filter out redundant effect config events that come too closely to avoid
applying effect on dsp buffer in vain.

Change-Id: If8915b1d068724d5976e1519ec57e3c08cf89c9d
CRs-Fixed: 562806, 563418
Merge remote-tracking branch 'cyanogen/cm-10.2' into cm-10.2