avcodec/mediacodecdec: wait on first frame after input buffers are full

The output_buffer_count==0 special case is no longer required, and
can cause spurious EAGAIN to surface to the user when input buffers
are filled up. Since the caller now knows if the decoder is accepting
new input (via current_input_buffer>=0), let the wait parameter
control whether we block or not.

Signed-off-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Matthieu Bouron <matthieu.bouron@gmail.com>