Do not flag unused parameters on _abstract_ methods.

I think this was introduced in 88a224c, where we suddenly start considering some _effectively_ private abstract methods.

PiperOrigin-RevId: 590895990

Author: graememorgan

core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java

@@ -706,7 +706,8 @@ private boolean isParameterSubjectToAnalysis(Symbol sym) {
       checkArgument(sym.getKind() == ElementKind.PARAMETER);
       Symbol enclosingMethod = sym.owner;
 
-      if (!(enclosingMethod instanceof MethodSymbol)) {
+      if (!(enclosingMethod instanceof MethodSymbol)
+          || isAbstract((MethodSymbol) enclosingMethod)) {
         return false;
       }
 

core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java

@@ -263,6 +263,9 @@ public void unusedParamInPrivateMethod() {
             "      System.out.println(i);",
             "    }",
             "  }",
+            "  private interface Foo {",
+            "    void foo(int a);",
+            "  }",
             "  public void main() {",
             "    test(1, 2);",
             "  }",