Use fixed size for gravatar images

This will make images from gravatar be always 400x400 px and
will fix the infamous crash in About AOKP.

Additionally use a different image for developers not found.

Change-Id: Id3d3e9ade6ec30986d87ae9a42e814552ecb644c
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>