android: use unified headers and llvm-c++ STL (x86) with NDK 16

android: use unified headers and llvm-c++ STL (x86) with NDK 16

gnustl (and others) are to be removed in future versions of the ndk
also bump gradle and build-tools to current versions along with it

arm unfortunately crashes with llvm-c++, so keep with gnustl for now/fix
that later

Change-Id: Ic794c3293b599b77ec48096bf3283a99c09cbb79
Reviewed-on: https://gerrit.libreoffice.org/45163
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>
Tested-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>