Add rat_switch_count_after_connected to VoiceCallSessionStats

It is counting the number of RAT switch after call was connected

Bug: 281605389
Test: atest VoiceCallsessionStatsTest
Test: manual testing RAT changes between LTE and IWLAN during
ringing/connected call
Change-Id: I7bffdeb54c772f6b05f68e5365db8730aba59fd5

Change-Id: I1ffc193a24c6a5a2396187d45787504fd8533770