Configurations/15-android.conf: fix implicit __ANDROID_API__ handling.

03ad7c009e16a233c733098db3169c560142ccd3 failed if one didn't pass
explicit -D__ANDROID_API__=N :-(

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7443)

(cherry picked from commit 71f2b3171e8b94b0fbdc7fab2d7ca4ea66edec2b)