libstagefright: Fixed PCM Offload format check

Small buffer hint is getting enabled for other formats
too due to in-correct checks in AudioTrack and AudioPlayer

Fixed the checks so that it gets enabled only for PCM
Offload formats

CRs-Fixed: 767824

Change-Id: I46cce8d66d0299c8ec8b8b12592c69cfe0abde18