BigPictureStyle: log when Icon.loadDrawable returns empty BitmapDrawable

Right now, if Icon.loadDrawable fails to decode the image file at the
end of an Icon URI, it returns a non-null BitmapDrawable with a null
Bitmap. I'm fixing this elsewhere, but in the meantime, log this as an
error.

Bug: 335878768
Flag: NONE only adds log messages
Test: manual
Change-Id: I69a75994dea55f4df88557bd693d7d14f1125c9d