libstagefright: Add support for custom LPA buffer size in legacy LPAPlayer

     - 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 breaks audio at in cm. So we have,
       to use buffer size of 512 to get audio working but it makes stuttering as mentioned.
       so add support for a custom buffer size so we can use 480 buffer size.

Change-Id: Ib4b5222b4b64d42288e0ad5c95910e385d172374