In O Developer Preview 1, to configure what autofill service the device should use (e.g., the one that you are writing), you would go into Settings > Apps & notifications > Default apps > Autofill app. Tapping on that entry would bring up a list dialog to choose the autofill app to use.
However, in O Developer Preview 2, there is no "Autofill app" entry in that screen, at least on a Nexus 5X and the x86 emulator:
The documentation, though, still refers to how we did it in ODP1.
I filed a bug report, but in the meantime... is there another way to configure an autofill service?