First, android is FOSS the same as Darwin (the system under iOS) is. Apple puts its proprietary drivers, ui, and other apps the same as android phone vendors do.
An second, Free Software/Open Source doesn't mean that you have to ship the phone with all the code anyone pushes. You have control of your repository. You can pay developers and only include the software the build. FOSS means that the user of the software has access to the source code, as well as other rights like modifying or redistributing it.
I see in a lot of discussion about free software some people say things like 'the code is open to everyone so you don't know what they can put there' as if there were no filter or anything