Add thread annotations (1/n)

Make it clear which thread a method should be called from. This should
make the code more readable and easier to reason about.

- @AnyThread annotates thread-safe methods.
- @WorkerThread annotates methods that should only run on the
  handler thread.

Bug: 145335013
Test: m

Change-Id: Iee986448dfdd32e2c0e898196da76072ed0af972