[android] Fix possibly race on undefined behavior

`runnable->iter = runnables.end()` assumes that `::end()` is a
`static` or `constxpr` and thread-safe. This is likely to be true
for most of the implementations. That said we cannot rely on
undefined behavior and we should assume the worse case scenario.

platform/android/src/run_loop.cpp

@@ -167,6 +167,7 @@ void RunLoop::Impl::removeRunnable(Runnable* runnable) {
 }
 
 void RunLoop::Impl::initRunnable(Runnable* runnable) {
+    std::lock_guard<std::recursive_mutex> lock(mtx);
     runnable->iter = runnables.end();
 }