Android: Record use of C++ by static libs in exported Android.mk files

When a `PREBUILT_STATIC_LIBRARY` uses C++ in its sources then the `.a`
file will have a link-time dependency on the C++ runtime libraries.
Android NDK r14 will add a way to give this information to the NDK build
system by adding a `LOCAL_HAS_CPP` setting to the `Android.mk` file.
Add this for exported static libraries that use C++.
1 job for android-export-has-cpp