libstagefright: Reset the decoder while performing seek.

- Observed noise when seeking to beginning of file for the recorded AMR clip.
- While performing seek, the AMR decoder states are not being reset. In AMR decoder, the current frame
  output is predicted using the states from previous frame. If these states are not cleared, it might
  cause noise when seek is performed.
- Fixed by resetting the decoder when seek is issued.

Change-Id: I412c362f554ba1613b440cf613349d2bdef0d5dd
CRs-Fixed: 263744