libstagefright: Correcting buffer size to be allocated from ashmem.

On 8650, all buffers are allocated as one chunk.
Individual buffers returned by ashmem are 32 byte
aligned so chunk size should be allocated keeping
in mind this 32 byte alignment.

Change-Id: Idc1928dfa61b5be155baa4af443fcb5d7aee81d9