nuplayer: Perform proper shutdown sequence for offload teardown

Extend shutdown sequence used for pcm offload to compress
offload as well. The current tear down sequence for compress
offload calls clear on the NuPlayerDecoderPassthrough object directly.
This is not guaranteed to prevent an encoded buffer from getting
queued into the Renderer. Going through a proper flush+shutdown
sequence helps prevent this (rare) scenario.

CRs-Fixed: 914308
Change-Id: Ie2040cbbd42b8c67bb53a7ef8a628faf1ab7acda