ahat: better handle bitmaps without pixel info available.

Don't print an exception nor show a broken link for bitmaps whose
pixel data is unavailable.

Bug: 30307315
Test: m ahat-test
Test: Open recent heap dump and view list of Bitmaps.
Change-Id: I4dda48f192ce6d4613decf52bf1f5e5100cdd846
1 file changed