Layer Lock: Fix incorrect kc parsing in QMK 0.19+.

This fixes #27

Layer Lock parses layer switch keycodes to know which layer they switch
to. Before this commit, this was still being done manually, and this
manual parsing became incorrect in QMK 0.19 when keycode encodings were
changed.

This commit revises to using the new keycode parsing helpers
`QK_MOMENTARY_GET_LAYER`, etc.

Big thanks to @sporkus for reporting this and identifying the fix.

features/layer_lock.c

@@ -1,4 +1,4 @@
-// Copyright 2022 Google LLC
+// Copyright 2022-2023 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -38,6 +38,17 @@ void layer_lock_task(void) {
 }
 #endif  // LAYER_LOCK_IDLE_TIMEOUT > 0
 
+// Handles an event on an `MO` or `TT` layer switch key.
+static bool handle_mo_or_tt(uint8_t layer, keyrecord_t* record) {
+  if (is_layer_locked(layer)) {
+    if (record->event.pressed) {  // On press, unlock the layer.
+      layer_lock_invert(layer);
+    }
+    return false;  // Skip default handling.
+  }
+  return true;
+}
+
 bool process_layer_lock(uint16_t keycode, keyrecord_t* record,
                         uint16_t lock_keycode) {
 #if LAYER_LOCK_IDLE_TIMEOUT > 0
@@ -59,21 +70,14 @@ bool process_layer_lock(uint16_t keycode, keyrecord_t* record,
 
   switch (keycode) {
     case QK_MOMENTARY ... QK_MOMENTARY_MAX:  // `MO(layer)` keys.
-    case QK_LAYER_TAP_TOGGLE ... QK_LAYER_TAP_TOGGLE_MAX: {  // `TT(layer)`.
-      const uint8_t layer = keycode & 255;
-      if (is_layer_locked(layer)) {
-        // Event on `MO` or `TT` key where layer is locked.
-        if (record->event.pressed) {  // On press, unlock the layer.
-          layer_lock_invert(layer);
-        }
-        return false;  // Skip default handling.
-      }
-    } break;
+      return handle_mo_or_tt(QK_MOMENTARY_GET_LAYER(keycode), record);
+    case QK_LAYER_TAP_TOGGLE ... QK_LAYER_TAP_TOGGLE_MAX:  // `TT(layer)`.
+      return handle_mo_or_tt(QK_LAYER_TAP_TOGGLE_GET_LAYER(keycode), record);
 
 #ifndef NO_ACTION_TAPPING
     case QK_LAYER_TAP ... QK_LAYER_TAP_MAX:  // `LT(layer, key)` keys.
       if (record->tap.count == 0 && !record->event.pressed &&
-          is_layer_locked((keycode >> 8) & 15)) {
+          is_layer_locked(QK_LAYER_TAP_GET_LAYER(keycode))) {
         // Release event on a held layer-tap key where the layer is locked.
         return false;  // Skip default handling so that layer stays on.
       }