BigPicture lazy loading

This CL covers the followign so far:
 - after inflation: read the image header (if we can) and set a placeholder or set the full image (if there is no header)
 - on expansion: read the full image and set it on the ImageView
 - on collapse: free the full image and set the placeholder again
 - throttle the jobs started by the expand/collapse calls

Gated by the statusbar flag bigpicture_notification_lazy_loading.

Bug: 283082473
Test: atest BigPictureIconManagerTest
Change-Id: I1f62516862a7bb8471b09ddb2c546ec696bf83a0