shill: Change Bus::is_connected to Bus::IsConnected

dbus::Bus::is_connected is now a virtual method and therefore needs to
be renamed to IsConnected as it's not a simple accessor anymore.

BUG=chromium:866704
TEST=Build succeeds
CQ-DEPEND=CL:1178682

Change-Id: Ie2f956f9d4ce92879e82f099391a38d6f5d1b7a5
Reviewed-on: https://chromium-review.googlesource.com/1180596
Commit-Ready: Sonny Sasaka <sonnysasaka@chromium.org>
Tested-by: Sonny Sasaka <sonnysasaka@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>