Add support for direct meters in generating P4 constants

Also, add imports for PI IDs only when needed in the constants file.

Change-Id: If1821a55a07d5f6874514174381be212188580e3
diff --git a/pipelines/fabric/api/src/main/java/org/onosproject/pipelines/fabric/FabricConstants.java b/pipelines/fabric/api/src/main/java/org/onosproject/pipelines/fabric/FabricConstants.java
index 00f5590..f78b9e4 100644
--- a/pipelines/fabric/api/src/main/java/org/onosproject/pipelines/fabric/FabricConstants.java
+++ b/pipelines/fabric/api/src/main/java/org/onosproject/pipelines/fabric/FabricConstants.java
@@ -16,14 +16,15 @@
 
 package org.onosproject.pipelines.fabric;
 
+import org.onosproject.net.pi.model.PiTableId;
+import org.onosproject.net.pi.model.PiMatchFieldId;
+import org.onosproject.net.pi.model.PiCounterId;
 import org.onosproject.net.pi.model.PiActionId;
 import org.onosproject.net.pi.model.PiActionParamId;
 import org.onosproject.net.pi.model.PiActionProfileId;
-import org.onosproject.net.pi.model.PiMeterId;
 import org.onosproject.net.pi.model.PiPacketMetadataId;
-import org.onosproject.net.pi.model.PiCounterId;
-import org.onosproject.net.pi.model.PiMatchFieldId;
-import org.onosproject.net.pi.model.PiTableId;
+import org.onosproject.net.pi.model.PiMeterId;
+
 /**
  * Constants for fabric pipeline.
  */