[Forensic] Refactor ForensicEvent

ForensicEvent will contain events of type SecurityEvent,
ConnectEvent, or DnsEvent. The contained event will be
denoted by the string mType, and constrained to values
EventType.

Bug: 369313906
Test: m -j ; atest ForensicServiceTest
Flag: android.security.afl_api
Ignore-AOSP-First: security feature
Change-Id: Ic87aca38631ef09d5cd0a43b0cbe5f5ff204af5b