Fix unterminated string causing flaky behavior.

Fixes shorebirdtech/shorebird#235
The root cause was we were not terminating the strings passed
for original_libapp_paths.  We used a Vector<char> but didn't
append a '\0' to the end before calling data(), so when the c/rust
side walked to find the end of the char* it sometimes found extra
characters at the end of the string unexpectedly.

This fixes to use a shorter (but probably equally dangerous?)
method of using c_str() which we had tried before but previously
had used c_str() on a temporary.  This attempt carefully uses
a reference to the string so the c_str()'s lifetime is tied to
the string in application_library_paths rather than a temporary
copy.

This is still slightly dangerous in that c_str() lives as long
as a std::string does, so long as it's not modified.

shell/platform/android/flutter_main.cc

@@ -104,33 +104,27 @@ void ConfigureShorebird(std::string android_cache_path,
   fml::CreateDirectory(fml::paths::GetCachesDirectory(), {"shorebird_updater"},
                        fml::FilePermission::kReadWrite);
 
-  AppParameters app_parameters;
-  app_parameters.release_version = version.c_str();
-  app_parameters.cache_dir = cache_dir.c_str();
-
-  // Converting the strings in `settings.application_library_path` to `char*`.
-  // We maintain a separate vector so that the lifetime of the `char*` is clear.
-  // If we used `c_str` then we would have to be very careful not to modify the
-  // string or have taken a copy of the string as `c_str` is only valid for the
-  // lifetime of the string.
-  std::vector<std::vector<char>> libapp_paths_storage;
-  std::vector<const char*> original_libapp_paths;
-  for (auto& path : settings.application_library_path) {
-    libapp_paths_storage.push_back(std::vector<char>(path.begin(), path.end()));
-    original_libapp_paths.push_back(libapp_paths_storage.back().data());
-  }
+  // Putting initialization into a block to make AppParameters lifetime clear.
+  {
+    AppParameters app_parameters;
+    app_parameters.release_version = version.c_str();
+    app_parameters.cache_dir = cache_dir.c_str();
+
+    // https://stackoverflow.com/questions/26032039/convert-vectorstring-into-char-c
+    std::vector<const char*> c_paths{};
+    for (const auto& string : settings.application_library_path) {
+      c_paths.push_back(string.c_str());
+    }
+    // Do not modify application_library_path or cstrings will be invalid.
 
-  app_parameters.original_libapp_paths = original_libapp_paths.data();
-  app_parameters.original_libapp_paths_size = original_libapp_paths.size();
+    app_parameters.original_libapp_paths = c_paths.data();
+    app_parameters.original_libapp_paths_size = c_paths.size();
 
-  // TODO: How do can we get the path to libflutter.so?
-  // The Rust side doesn't actually use this yet.  The intent is so
-  // that Rust could hash it, or otherwise know that it matches the
-  // version the patch is intended for, but currently that's not
-  // implemented.
-  app_parameters.vm_path = "libflutter.so";
+    app_parameters.vm_path = "libflutter.so";  // Unused.
 
-  shorebird_init(&app_parameters, shorebirdYaml.c_str());
+    // shorebird_init copies from app_parameters and shorebirdYaml.
+    shorebird_init(&app_parameters, shorebirdYaml.c_str());
+  }
 
   FML_LOG(INFO) << "Starting Shorebird update";
   shorebird_update();