Move Signal.java to "solution" file in Secret Handshake (#2825)

Previously, it was listed as an "editor" file, which makes the file
"read only" (i.e. students are not allowed to modify this file). But
this contradicts our mentoring notes, which encourage students to make
changes. Moving to "solution" allows students to make modifications.

Config.json docs: https://exercism.org/docs/building/tracks/config-json
Mentoring notes: https://github.com/exercism/website-copy/blob/main/tracks/java/exercises/secret-handshake/mentoring.md

Fixes #2824

Author: kahgoh

exercises/practice/secret-handshake/.meta/config.json

@@ -21,17 +21,15 @@
   ],
   "files": {
     "solution": [
-      "src/main/java/HandshakeCalculator.java"
+      "src/main/java/HandshakeCalculator.java",
+      "src/main/java/Signal.java"
     ],
     "test": [
       "src/test/java/HandshakeCalculatorTest.java"
     ],
     "example": [
       ".meta/src/reference/java/HandshakeCalculator.java"
     ],
-    "editor": [
-      "src/main/java/Signal.java"
-    ],
     "invalidator": [
       "build.gradle"
     ]