Android: Do not pass non-existent Clang -std flags
Android NDK r16b comes with
Android clang version 5.0.300080 (based on LLVM 5.0.300080)
Although it claims version 5, it does not support the
-std=c++2a flags that upstream Clang does.
Android NDK r17-beta1 comes with
Android (4639204 based on r316199) clang version 6.0.1
that does have the flags.
Follow up to !1892 (merged).