stagefright-plugins: Return real bitwidth in getParameter call

 * ..and store the real parameters during setParameter.

Change-Id: I170b283da649303e6c016458a3bfebd3431b5c42
stagefright-plugins: Add a couple of sample format exceptions

 * DTS and AC3 need to output 24-bit samples if supported.

Change-Id: I84174994d568645e7c7c593901fff30b998bf92a