nuplayer: Fix incorrect state handling while shutting down a decoder

This patch fixes a race condition between a flush issued by NuPlayer
as part of reset and the handling of an offload teardown message.

If the offload teardown message gets handled while NuPlayer is waiting for
a flush complete message, the audio decoder is cleared while the decoder
is being flushed. On receiving a flush complete message,
NuPlayer tries to initiate shutdown on an already cleared decoder
leading to a crash

To avoid this, reject all flush requests if the decoder is
already shutdown or pending shutdown.

CRs-Fixed: 821831
Change-Id: I4c6f2c8d346931006acdd3b522310ed234ab72f1