apexd: implement session rollback

So far only session in SessionState::ACTIVATED can be rolled back. As
apexd-level rollbacks only cover scenarios when RollbackManager is not
yet present to issue a rollback request.

Rollback can happen in the following situations:
- Watchdog-initiated rollback (ro.init.updatable_crashing is true);
- abortActiveSession() is called when session is in the "ACTIVATED"
  state;

Rollback itself is as simple as:
1. cache permissions of /data/apex/active;
2. delete content of /data/apex/active;
3. rename /data/apex/backup to /data/apex/active;
4. restore permissions;

Bug: 123622800
Test: apexservice_test
Change-Id: I333698687f5ab06bca2d24e8f9a6fb043aea705e

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