Rename PricingEngine to PremiumPricingEngine

This properly reflects the fact that other, separate things will now
be responsible both for EAP and for per-TLD custom pricing.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=124558165

javatests/google/registry/model/registry/label/PremiumListTest.java

@@ -74,7 +74,7 @@ public class PremiumListTest {
     persistResource(
         new Registry.Builder()
             .setTldStr("ghost")
-            .setPricingEngineClass(StaticPremiumListPricingEngine.class)
+            .setPremiumPricingEngineClass(StaticPremiumListPricingEngine.class)
             .build());
     assertThat(Registry.get("ghost").getPremiumList()).isNull();
     assertThat(getPremiumPrice("blah", "ghost")).isAbsent();