ART: Flush ostream less frequently in GraphVisualizer

We have seen Checker tests timing out on debug-GC configurations after
having switched to Optimizing because its GraphVisualizer makes too
many syscalls which the configuration keeps track of.

This patch replaces std::endl with "\n" across GraphVisualizer so as
to not flush the stream after every line of output.

Bug: 27826765
Change-Id: I5e3f1e92f8a84f36d324d56945e2d420b2d36a5d