OK, there are a couple of things:
Google had very little to do with what is on your phone. HTC was the one that made it, and to a lesser extent T-Mobile. Blame them, not Google. That is like me blaming The Linux Foundation ever ytime a kernel update in Ubuntu causes a problem.
Also, factory reset. What you are probably experiencing is that configuration files and other system data that is from the old system somehow managed to migrate to the new system post update. In theory this should never happen, but perhaps a cleanup script faulted during update, or something.
Lastly, you do not need ATK or any task manager save for the one built into the OS/kernel. I keep ATK on my phone, but only for killing unstable or otherwise misbehaving apps (I play around with a lot of betas and my own novice coding, so there are bugs; for normal use ATK is unnecessary).