No. It's not because it's open source. it's because it's open, i.e. you can install any app you want, from any source. You don't need to download your apps from the Google Play Store. Unlike Apple, where the only source is the iTunes App Store, and you don't get any choice whatsoever.

One of the biggest problems in Android is the amount of third party attacks, for example Samsung's Knox, designed to IMPROVE security, has a very major flaw which actually makes their phones highly vulnerable. (which was actually used in the TV Show Mr Robot to hack the FBI) - thus having a non-samsung phone actually improves your security simply because of that attack.

It's thanks to companies like HTC, Samsung, and Sony filling up their android phones will all their vendor crapware that opens up whole new vectors of attacks. Every now and then we do get attacks that directly damage android itself, such as this, which is somewhat Qualcomm's fault, not even Android's fault in it's own.