Fix compilation errors with Android NDK

[xfan1024]

I compiled cpprestsdk with android-ndk-r16b and got some error information. I tried to fix them and it's works for me.

1. std::to_string not support by Android NDK with gnustl. Using utility::conversions::details::to_string instead of std::to_string when __ANDROID__ was defined.
2. if BOOST_ASIO_HAS_STD_CHRONO defined, boost::chrono::microseconds cannot cast to boost::asio::steady_timer::duration.

Scanning dependencies of target cpprest
[ 3%] Building CXX object Release/src/CMakeFiles/cpprest.dir/http/client/http_client_asio.cpp.o
/home/xiaofan/workspace/casablanca/Release/src/http/client/http_client_asio.cpp:1873:21: error: no matching member function for call to 'expires_from_now'
m_timer.expires_from_now(m_duration);
~~~~~~~~^~~~~~~~~~~~~~~~
/home/xiaofan/workspace/casablanca/Build_android/build/Boost-for-Android/build/out/x86/include/boost-1_70/boost/asio/basic_waitable_timer.hpp:605:15: note: candidate function not viable: no known conversion from
'boost::chrono::microseconds' (aka 'duration<long long, ratio<1LL, 1000000LL> >') to 'const boost::asio::basic_waitable_timer<std::chrono::_V2::steady_clock,
boost::asio::wait_traitsstd::chrono::_V2::steady_clock, boost::asio::executor>::duration' (aka 'const duration<long long, ratio<1, 1000000000> >') for 1st argument
std::size_t expires_from_now(const duration& expiry_time)
^
/home/xiaofan/workspace/casablanca/Build_android/build/Boost-for-Android/build/out/x86/include/boost-1_70/boost/asio/basic_waitable_timer.hpp:577:12: note: candidate function not viable: requires 0 arguments, but
1 was provided
duration expires_from_now() const
^
/home/xiaofan/workspace/casablanca/Build_android/build/Boost-for-Android/build/out/x86/include/boost-1_70/boost/asio/basic_waitable_timer.hpp:637:15: note: candidate function not viable: requires 2 arguments, but
1 was provided
std::size_t expires_from_now(const duration& expiry_time,
^
/home/xiaofan/workspace/casablanca/Release/src/http/client/http_client_asio.cpp:1884:25: error: no matching member function for call to 'expires_from_now'
if (m_timer.expires_from_now(m_duration) > 0)
~~~~~~~~^~~~~~~~~~~~~~~~
/home/xiaofan/workspace/casablanca/Build_android/build/Boost-for-Android/build/out/x86/include/boost-1_70/boost/asio/basic_waitable_timer.hpp:605:15: note: candidate function not viable: no known conversion from
'boost::chrono::microseconds' (aka 'duration<long long, ratio<1LL, 1000000LL> >') to 'const boost::asio::basic_waitable_timer<std::chrono::_V2::steady_clock,
boost::asio::wait_traitsstd::chrono::_V2::steady_clock, boost::asio::executor>::duration' (aka 'const duration<long long, ratio<1, 1000000000> >') for 1st argument
std::size_t expires_from_now(const duration& expiry_time)
^
/home/xiaofan/workspace/casablanca/Build_android/build/Boost-for-Android/build/out/x86/include/boost-1_70/boost/asio/basic_waitable_timer.hpp:577:12: note: candidate function not viable: requires 0 arguments, but
1 was provided
duration expires_from_now() const
^
/home/xiaofan/workspace/casablanca/Build_android/build/Boost-for-Android/build/out/x86/include/boost-1_70/boost/asio/basic_waitable_timer.hpp:637:15: note: candidate function not viable: requires 2 arguments, but
1 was provided
std::size_t expires_from_now(const duration& expiry_time,
^
2 errors generated.
Release/src/CMakeFiles/cpprest.dir/build.make:335: recipe for target 'Release/src/CMakeFiles/cpprest.dir/http/client/http_client_asio.cpp.o' failed
make[2]: *** [Release/src/CMakeFiles/cpprest.dir/http/client/http_client_asio.cpp.o] Error 1
CMakeFiles/Makefile2:108: recipe for target 'Release/src/CMakeFiles/cpprest.dir/all' failed
make[1]: *** [Release/src/CMakeFiles/cpprest.dir/all] Error 2
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2

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