When the download is done, your browser will remain in memory in an inactive state. Apps that don't need to dobackground processing, like BillingAccountService, just sit there, using no CPU and no battery until they are needed by the system, or by you, to do something.
WHY DOES ANDROID NEED TO KEEP APPLICATIONS IN MEMORY OR RAM?Because it's likely that if you recently used an app or a service, you will use it again soon. If it's in memory, the OS can simply request it quickly to redraw it's screen. If you've killed it or used a task manager to clear recent apps from memory, the OS will need to copy from storage (ROM) back into RAM, start it up, wait for it to allocate any resources it needs, and then redraw the screen again. That takes longer, and uses more battery than if you had just let it run. If you start a new app that needs memory an inactive app you are not using will be closed and the os will allocates the ram to the new open and running application.
What you need to understand is that most apps in memory/ram aren't really "running." They're in an inactive state, using no cpu, and no battery.
The problem with task killers is that:
Android doesn't know what they do, so it will allocate more MEMORY to them, and third party task managers tend to kill processes that Android needs to run. Google has spent thousands of man hours making Android manage memory and applications effectively and efficiently and no android apps either free or paid can help you manage your phone better.
Installing Taskmanger on your phone is not necessary Let Android do what it was designed to do. Your phone, will be faster and work better without third party task killers/managers.