Add missing "unwind" and "atomic" libraries needed for this combination.
See `${ndk}/sources/cxx-stl/llvm-libc++/libs/armeabi/libc++.a` for the
libraries the NDK uses.
Issue: #16380
We use `-Werror` in the Android test builds to make sure there are
no warnings that we care about (e.g. unused flags). However, the
NDK r13 tools produce a warning about their own builtins:
```
<built-in>: In function 'float abs(float)':
<built-in>: warning: conflicts with previous declaration here [-Wattributes]
```
Suppress this warning so that we can continue using `-Werror` but
tolerate these warnings.
Brad King