setPolicy method
Sets the policy of a to mode with holder, raises its version and the permission
version and stamps billing_mode_changed_at.
Implementation
void setPolicy(FakeAgent a, String mode, String? holder) {
a
..billingMode = mode
..credentialHolderId = holder
..version += 1
..billingModeChangedAt = world.stamp()
..mismatchAtHeartbeat = false;
world.permissionVersion++;
}