libstagefright: Ignore other sniffers if current sniffer returns positive confidence

-MP3 sniffer functions takes around 400ms for the playback
of most of the clips.
-This was causing a startup delay of 1.5 seconds on playback
-Increasing the value of confidence for amr,wav,ogg,mpeg4 formats
to 0.6 so that when they are detected, we ignore the sniffMP3
function.

Change-Id: I08acf28a63c8ae7b1b69e03f85fa9065a52c649e