Android: Detect API version of standalone toolchain with unified headers
A standalone toolchain with unified headers keeps the `__ANDROID_API__` macro in the `bin/clang` launcher instead of in `api-level.h`. Issue: #16954
A standalone toolchain with unified headers keeps the `__ANDROID_API__` macro in the `bin/clang` launcher instead of in `api-level.h`. Issue: #16954