Change RESTORE BillingEvents to have 1-year periods

This simplifies calculating the overall invoice by giving RESTORE fees a
period equal to the period of the associated RENEW (1 year). Older
BillingEvents will not be backfilled, and will have periodYears = null.

Invoicing and business both agree this is a valid representation, since RESTORE fees are intrinsically tied to the 1-year RENEW it's associated with.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188041777

java/google/registry/model/billing/BillingEvent.java

@@ -305,10 +305,12 @@ public abstract class BillingEvent extends ImmutableObject
             Sets.immutableEnumSet(
                 Reason.CREATE,
                 Reason.RENEW,
+                Reason.RESTORE,
                 Reason.TRANSFER);
         checkState(
             reasonsWithPeriods.contains(instance.reason) == (instance.periodYears != null),
-            "Period years must be set if and only if reason is CREATE, RENEW, or TRANSFER.");
+            "Period years must be set if and only if reason is "
+                + "CREATE, RENEW, RESTORE or TRANSFER.");
         checkState(
             instance.getFlags().contains(Flag.SYNTHETIC)
                 == (instance.syntheticCreationTime != null),