build: fixes for NDK r15b

- switch to clang to resolve various ndk-build gcc errors introduced in NDK r14b
- suppress warnings in aapt, libandroidfw and libcutils since clang is much noisier than gcc
- clang is more rigid so work around cpp file compilation within libcutils
- a link to libatomic.a is now required
- target platform currently cannot go above android-21 due to libc.a not being up to date