stagefright: Fix offload fallback for 24bit

Change I398d964a3c5b18f58fcc79376bc29222e796badc disables 24bit
playback on non-offloadable tracks, but tries to read the format
after clear()ing the audio source. Fix the operation order to
do the disable before resetting the source.

Change-Id: Idbbd5e21f9e8fc357a494fabe9028331ebfd2ac4