ANDROID: driver core: Allow a device to wait on optional suppliers

Before this change, if a device is waiting on suppliers, it's assumed
that all those suppliers are needed for the device to probe
successfully. This change allows marking a devices as waiting only on
optional suppliers. This allows a device to wait on suppliers (and link
to them as soon as they are available) without preventing the device
from being probed.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Bug: 142657042
Change-Id: I86a764b67804ed889f17c55632e25d46a72912a8