We cannot guarantee that Android is designed to be safe, the format was designed to give more freedom. When people talk about 90% of malware for Android, they must of course take into account the fact that it is the most popular operating system in the world. If I had a company dedicated to malware, I would also be addressing my attacks on Android.
Malware authors may be writing a lot of malware for Android, but they're not very good at it - less than 0.001% of all application installations on Android (in and outside of Google Play) penetrate Android's security.
In other words, this is a complete non-issue - no matter how often antivirus companies and certain bloggers drum it up.