audiopolicy: Revert all session callback patches.

 * This has been rearchitected in a better way, as this
   feature turns out to be more difficult than it seems.
 * Reverting all of this stuff and rolling it into a single commit.

This reverts commit c27a16c3.
This reverts commit 32ef0556.
This reverts commit 489c9fb6.
This reverts commit a4123803.
This reverts commit e13b58b9.
This reverts commit 47f8c730.
This reverts commit 0479d7c7.

Change-Id: Iaed9f198d806aa414c95960713e8187c98db248b