Encoder now writes the binary symbol for show_existing_frame to
indicate the value of reset_decoder_state. The default value now
is set as zero regardless. This enables the bitstream match with
the decoder.

With this patch when fwd-kf is enabled, one additional bit is
signaled at the frame header for show_existing_frame, but without
any coding performance impact.

Change-Id: Ib0137e6a513011c0ac9506837d32e9ad405508af