pipewire: run with the System::Pipewire smack label

pipewire: run with the System::Pipewire smack label

Pipewire shares memory with its clients using open file
descriptors (memfd or shared memory file) which are created within
pipewire and therefore they have the same smack label as the
pipewire process. Clients must be able to read and write to this
memory, therefore they need rw acess to that smack label.

Since all AGL apps have only write access to the System label,
we need to use a different smack label which can be granted rw
access from the applications that need to use audio.
"System::Pipewire" is chosen here to follow the "System::Sub"
pattern that is described in the documentation.

Bug-AGL: SPEC-2554

Change-Id: I81cbf82adfde3ef4f67872bd91293370339b18d7
Signed-off-by: George Kiagiadakis <george.kiagiadakis@collabora.com>