Make isAppBad() lock-free

We trade off increased cost of (rare) mutations of the set of "bad" apps
in order to make the core lookup "is this specific app bad?" lock-free.
Locking discipline turned out to be a dominant cost in a very hot code
path within the running system, even uncontended.

Bug: 175339368
Test: tbd
Change-Id: I8803a177d46b01b86897f1e8d0966eb2452849fd