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)

Configurations/15-android.conf

@@ -73,6 +73,8 @@
                 die "no $ndk/platforms/android-$api" if ($#platforms < 0);
 
                 $sysroot = "@platforms[$#platforms]/arch-$arch";
+                $sysroot =~ m|/android-([0-9]+)/arch-$arch|;
+                $api = $1;
             }
             die "no sysroot=$sysroot"   if (!-d $sysroot);