Split Timer into OnceTimer and RepeatingTimer

Follow the naming convention in libbase, where Callback was split into
OnceCallback, and RepeatingCallback, to make it obvious how many times
callback is expected to be called.

Test: unit tests were split from TimerTest into OnceTimerTest and
RepeatingTimerTest
Bug: 110303473
Change-Id: Ibbb9bb74a2e6e1a0f0f74c6ec44f63c5a4a1537f