Implement staged directory clean up for staged install

Staged files should be deleted from /data/staging once installation
is complete, regardless of failure or success. If a session fails
pre-reboot verification, then it can be deleted before reboot.

Bug: 123624108
Test: manual testing
      - create two apex files; aosp signed and dev signed
      - flash device with internal master
      - aosp signed apex file will fail pre-reboot verification
      - check that files are deleted in /data/staging
      - repeat with multiple package
      - dev signed package passes pre-reboot verification
      - on reboot, files should be deleted from /data/staging

Change-Id: I44a6cc72de26ad0216942623edb6e279c628d58d