Remove Checker Framework; add NullAway, Picnic Error Prone Support.

Change-Id: I6558f0b4db0f9a192c18bbe45e2eaf10595bc666
diff --git a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Summary.java b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Summary.java
index 150ab00..2b8096d 100644
--- a/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Summary.java
+++ b/jgvariant-ostree/src/main/java/eu/mulk/jgvariant/ostree/Summary.java
@@ -4,9 +4,10 @@
 
 package eu.mulk.jgvariant.ostree;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 import eu.mulk.jgvariant.core.Decoder;
 import java.nio.ByteOrder;
-import java.nio.charset.StandardCharsets;
 import java.util.List;
 
 /**
@@ -58,7 +59,7 @@
     }
 
     private static final Decoder<Entry> DECODER =
-        Decoder.ofStructure(Entry.class, Decoder.ofString(StandardCharsets.UTF_8), Value.decoder());
+        Decoder.ofStructure(Entry.class, Decoder.ofString(UTF_8), Value.decoder());
 
     /**
      * Acquires a {@link Decoder} for the enclosing type.