SSIN: Use the application's context to load icon resources.

SSIN = Smart Suggestions in Notifications.
When posting a notification with contextual/smart actions app developers
supply their own icons for their actions. These icons can reference
resources from within the application package. Therefore we need to use
a Context from that package to load icon resources.
Before this CL we were using the SystemUi context which doesn't have
access to the same resources as the app context - meaning we were not
able to load the icon the app developer was providing us (unless they
provided an Icon instance rather than just a resource ID for their icon).

Bug: 137044913
Test: atest SmartReplyViewTest
Test: manual - create a notification with a
contextual action whose icon is referenced through a resource ID (not an
Icon instance). Ensure the icon is shown correctly in the notification.
Change-Id: Ia6d7b3689c46b65558437e00a947ba3cd330e4ca