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++17 or -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).