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 and port flush
completion is notified to the decoder.

CRs-Fixed: 500851
Change-Id: Ia8480b88521abc09c8b01d6052df4730a0359518