[CDM perm sync] Backup and restore permissions

Main changes:
1. Backup/restore permissions into/from CompanionMessage. ~95% of the
B+R code is copied from PermissionController and minimal changes have
been made to accomodate the CDM perm sync case. The main logical changes are
near the commented blocks in the files.
2. Message pagination

Bug: 228228540

Test: Manually tested on CDM test app, checked the output proto bytes.
Restore hasn't been tested, will test when the test app is done.

Change-Id: I21cdb6106b4237b72ea798ba3f206db5ee136916