Refactor "isMutable" -> "isFrozen"

This helps readability, particularly since we also have "isImmutable" for SkylarkValues and in EvalUtils. I considered changing those to isFrozen as well, but we can leave it as-is since the terminology of freezing doesn't necessarily apply to non-Freezable things.

Also rephrased some javadoc.

RELNOTES: None
PiperOrigin-RevId: 155090987
diff --git a/src/main/java/com/google/devtools/build/lib/syntax/UserDefinedFunction.java b/src/main/java/com/google/devtools/build/lib/syntax/UserDefinedFunction.java
index 4b81e22..3712edc 100644
--- a/src/main/java/com/google/devtools/build/lib/syntax/UserDefinedFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/syntax/UserDefinedFunction.java
@@ -51,7 +51,7 @@
   @Override
   public Object call(Object[] arguments, FuncallExpression ast, Environment env)
       throws EvalException, InterruptedException {
-    if (!env.mutability().isMutable()) {
+    if (env.mutability().isFrozen()) {
       throw new EvalException(getLocation(), "Trying to call in frozen environment");
     }
     if (env.getStackTrace().contains(this)) {