Make BigPictureIconManager reload the image on each icon update

This way we reload the URI every time a BigPicture Notification is
reinflated, which means that when the system theme changes the app is
given an opportunity to provide a version of the image for the current
system theme. This matches the behaviour before the lazy loading.

Bug: 283082473
Test: atest BigPictureIconManager
Change-Id: I54cf91bd42f0eb760774caf88005150f0a4cec3d