Call adjustSpacingForImage only when showing the image

The fix from Ife591964a0b83377889d263eed0d23423fa4493b unfortunately
made adjustSpacingForImage be called a bit too eagerly - so when we had
a large icon, but no messaging image, the margins calculated in
Notification.java would be overridden.

Fix: 433674096
Flag: android.app.notifications_redesign_templates
Test: manually test group conversations with and without large icons and
image messages (and combinations thereof)

Change-Id: I5e9daf60818727b1e6c1f12fc8ea47b775e1d3a9