FFMPEG Plugin: Refactor profiling of audio decoding.

Profiling of audio decoding is done with local variables instead of
global variables. An audio frame contains usually hundreds of audio
samples and you really wanne have the profiling information for -every-
audio frame due to the pure number of audio samples it represents.
FFMPEG Plugin: Reorder method sequence.

Makes it easier to navigate the code with Pe.

No functional change intended.