fix missing std::to_string during Android build, android rpc test script

[eqy]

currently the Android build breaks on my end with:

/sampa/home/eqy/tvm/apps/android_rpc/app/src/main/jni/../../../../../../include/../src/runtime/thread_pool.cc:77:31: error: no member named 'to_string' in namespace 'std'
err += "Task " + std::to_string(i) + " error: " + par_errors_[i] + '\n';

switching the APP_STL seems to fix:
https://stackoverflow.com/a/27563132

also remove broken import in android rpc test