Modify Settings Spa field and enums

- change the enum name ScenarioCategory to SessionType.
- change the field name category to session_type
  in the SettingsSpaReported and type to SessionType.
- add two new session types SESSION_UNKNOWN and SLICE_TYPE.

Eldar: http://eldar/317660610

Bug: 253979024
Bug: 264325337
Test: Using statsd_testdrive
Change-Id: Ife86e74e1477ea85ec20f897bc566816ad4061af