policy_hal: Update offload restrictions

 * Fix the PCM offload duration checks
 * Reject offloading for A2DP and USB devices in order to
   hit the fail-fast path in Stagefright- this allows transparent
   fallback to 16-bit PCM with resampling when necessary.

Change-Id: I26d1fd9d92f267c6875e98ef490e8d90bb442813