libstagefright: Fix crash in FLAC decoder during fallback to sw decode

- Playback of FLAC clips fail over A2DP when PCM offload is enabled.
- As offload is not supported for A2DP, playback falls back to software
  decode during which the source (FLACDecoder) is stopped and the
  buffers allocated to the decoder are freed. These buffers are not
  re-allocated before attempting to decode data due to which
  the decoder crashes.
- Allocate memory while starting the decoder instead of allocating
  it during initialization.

CRs-Fixed: 743183
Change-Id: I05d9d2328ac8b959ae112699475d80556a4738b5