libstagefright: Add support for custom LPA buffer size

     * On msm7x27a with bootloader heap for lpa audio,during audio playback with,
       DSP Manager effects enabled there is a lot of stuttering with the default buffer size.
       with bl heap we need buffer size of 480, but that caused no audio at all in CM because.
       So up until we have to use buffer size of 512 to get audio working at all,
       but unfortunately it causes stuttering as mentioned. Hence add support
       for a custom buffer size so we can use 480 value and enjoy no stuttering.

    Change-Id: Id4d090bc6cf90782c24f3832a35ddeca43bd72fc

Change-Id: Ib4b5222b4b64d42288e0ad5c95910e385d172374