Revert "hal: Don't mess with primary output"

This reverts commit fc56491ce39b1ee499c69ec25f88d2e3a0fe2b72.

Change-Id: Ifa0b364d01e5057e845572e9c3c011fdbbf02b2b
hal: Support the audio amplifier hook

* Original legacy HAL commit:
  Ib236598a5888b2af19bcfb81e285f644a0e84c0d
* Example: http://review.cyanogenmod.org/38221

Change-Id: Ic944a9f7059c78b79322dae9c787cdd8bb029cff
hal: Fix mismerge

 * Should not have been removed.

Change-Id: Icfd71d7621e99d0a164054b3dceff208f80cd28f