Added fine-grained tracing to UserController.startUser()

Also refactored SystemServer.BOOT_TIMINGS_TRACE_LOG into a helper class, so it's not permanently
allocated.

Bug: 133242016
Test: manual verification
Test: atest CtsDevicePolicyManagerTestCases:com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_StartInBackground

Change-Id: Id059d4d777bfc705e510d157930aefc7757d8cf8