[core] Remove util::Thread

[tmpsantos]

Umbrella issue for the tasks needed for removing util::Thread.

Our old threading model will now be replaced with the new Actor model merged at #6337. Currently a few places still use util::Thread and needs to be refactored:

• DefaultFileSource for non-blocking networking. [core] Remove util::Thread #6425, Convert DefaultFileSource and dependents to actor model #7678
• AssetFileSource for non-blocking disk I/O.
• Android's RunLoop for timer on the main thread (because Looper doesn't have the concept of timer callbacks)
• Unit tests stressing some of our thread-safe classes, like AsyncTask

/cc @jfirebaugh