Make all injector-getters abstract in public types

[lptr]

We use two different syntaxes for getter-injection in tasks and elsewhere:

    @Inject
    protected BuildOperationRunner getBuildOperationRunner() {
        throw new UnsupportedOperationException();
    }

and:

    @Inject
    protected abstract BuildOperationRunner getBuildOperationRunner();

We should refactor the former ones to the latter, as it's more concise. The change is technically a breaking change, though it's unlikely to affect code out there. Still, it'd be prudent to do it in a major release.