SF: Remove ConnectionHandle concept in Scheduler

This abstraction never made sense, and only incurs pointless locking and
hash map lookups. This is a step toward replacing the two EventThreads
with a single EventDispatcher.

Start the Scheduler's timers after initialization. This solves the same
bug as Ib2617b914145bc4180cc7ca27203c59dbd625c94, which avoided crashes
without fixing the root cause of the race.

In a few cases, initScheduler indirectly calls Scheduler functions that
attempt EventThread dispatch before EventThread creation, so skip those
EventThread accesses using hasEventThreads for now. Further refactoring
will untangle circular dependencies and remove hasEventThreads.

Bug: 241285191
Bug: 241285945
Test: presubmit
Change-Id: I89b93b47540ab24bb5675edc79370f38bc48114f