Add registering_seconds and unregistered_seconds to ImsRegistrationStats

Bug: 284220679
Test: atest ImsStatsTest
Test: atest com.android.internal.telephony.metrics
Test: manual test to check ims registering and unregistered duration
while turning airplane mode on and off

Change-Id: Ia3de3053a9a07c5d7ca8e610533a22bea01db12f