Expand Android platform build coverage

src/assets/brush_package.cpp

@@ -19,7 +19,8 @@ namespace {
         return false;
     }
 
-    for (const unsigned char ch : value) {
+    for (const char raw : value) {
+        const auto ch = static_cast<unsigned char>(raw);
         if (std::isalnum(ch) == 0 && ch != '_') {
             return false;
         }