PatchPanel: store "downstream" patches for a device

When the system inserts an intermediate processing module between
audio flinger and hardware HAL, and they are connected via a
software patch, the threads of this patch need to receive
metadata passed to the thread serving the "upstream" device of
the processing module.

PatchPanel stores an association between "upstream" streams
and the software patch. This allows audio flinger to access
the patch threads.

For more info, see the comments in PatchPanel.h

Bug: 63901775
Test: manual
Change-Id: I60e578c37a1bc7385daf32d379ea143120584a31