Add /data/system/car dir

- car service should use this directory for storing non-user sensitive
  system data.
- Update CarStorageMonitoringService to use it instead of user 0 data.
- Update sepolocy to allow this.

Bug: 124232192
Test: confirm that the dir is generated after boot up

Change-Id: I12051aebf37a185b97a62ccdb93c6db3c9bcde9b
11 files changed