By default, when building with --enable-accounting the bit accounting
 code will collect statistics for every frame while decoding.
Collecting statistics can slow down decode time and we would eventually
 like to enable the CONFIG_ACCOUNTING flag by default.
This patch adds a runtime flag so that bit accounting statistics are
 only collected when actually needed.

Change-Id: I25d9eaf26ea132d61ace95b952872158c9ac29e7