Make sure we can properly shutdown even if

a) one of the two decoders has a pending discontinuity
b) the renderer holds on to all output buffers for that decoder
c) the renderer is paused

if all three conditions are met the decoder won't ask for more input data
and therefore never see the discontinuity.

To avoid this we briefly resume the renderer just before shutting down.

Change-Id: I9e08af2a1eb4298d1cd00497d6aa33f4ad184e9a
related-to-bug: 5655016