Fixed registered_times in ImsRegistrationStats

- This field is to calculate the number of times it is registered.
- It was being misused as it was divided like other duration.
- mark 16 as reserved
- add registered_time field as 17

Bug: 312808192
Test: atest ImsStatsTest
Test: atest PersistAtomsStorageTest
Change-Id: Ib1df4a8a60abfa9e833f77dcbb5e2a9bbf83de87