update_engine: non interactive update in update need reboot state

update_engine running into indefinite loop for allowing the non
interactive update attempt when the current update attempt state is in
UPDATED_NEED_REBOOT. Following function calls are causing the loop
Non interactive -> CheckForUpdate -> ScheduleUpdates -> OnUpdateScheduled
-> Update -> PingOmaha -> ScheduleUpdates

Modified the code to stop running the periodic update checks when the current
state is in UPDATED_NEED_REBOOT. Starting the periodic update checks
when the state changed from UPDATED_NEED_REBOOT to IDLE.

BUG=b:130061952, chromium:960828
TEST=Performed the back to back interactive and non interactive update
attempts in all state changes.

Change-Id: Id1dda5edb9340a96e4dd3f32157b0b74d87f4ec0