Refactor PermissionDecision to extend generic event

Introduce an abstract superclass, PermissionEvent, that we can use with
storage classes to persist permission event info. This CL just
introduces the PermissionEvent class and refactors some of the class
members.

Bug: 205642821
Test: builds
Change-Id: If75e2cb0919580be32b61c0bb36db61a4be2d149