AIDL effect: add draining state support

 - effects can transit to DRAINING state with STOP command and continue
   the fade out processing
 - add draining in the audio eraser effect as an example
 - update VTS to support draining

Flag: EXEMPT bugfix
Bug: 379776482
Test: --test-mapping hardware/interfaces/audio/aidl/vts:presubmit

Change-Id: I2ca25cd085d1b6ae6cf8b0d1b58cd713aef0f7e5
Merged-In: I2ca25cd085d1b6ae6cf8b0d1b58cd713aef0f7e5