Properly convert milliseconds to seconds (#767)

* Properly convert milliseconds to seconds

Currently, a value expressed in milliseconds is directly assigned to a variable
which is supposed to store seconds and it's later converted to mills again.
Thus, convert properly upon assignment

* Update readme.md

Author: pawelsnk

release_build_files/readme.md

@@ -584,7 +584,9 @@ code.
         ([#745](https://github.com/firebase/firebase-cpp-sdk/pull/745))
     -   Messaging (Android): Fixed crash during initialization.
         ([#760](https://github.com/firebase/firebase-cpp-sdk/pull/760))
-
+    -   Remote Config (Desktop): Fixed cache expiration time value used by
+        `RemoteConfig::FetchAndActivate()`.
+        ([#767](https://github.com/firebase/firebase-cpp-sdk/pull/767))
 
 ### 8.7.0
 -   Changes

remote_config/src/desktop/remote_config_desktop.cc

@@ -140,7 +140,8 @@ Future<bool> RemoteConfigInternal::FetchAndActivate() {
       future_impl_.SafeAlloc<bool>(kRemoteConfigFnFetchAndActivate);
 
   cache_expiration_in_seconds_ =
-      config_settings_.minimum_fetch_interval_in_milliseconds;
+      config_settings_.minimum_fetch_interval_in_milliseconds /
+      ::firebase::internal::kMillisecondsPerSecond;
 
   uint64_t milliseconds_since_epoch =
       std::chrono::duration_cast<std::chrono::milliseconds>(