Delete orphaned staging directories for staged session on reboot

Bug: 173132101
Test: manually created folders in /data/app-staging and observed they were deleted on reboot
Change-Id: If840f35245c2d049401d0d2f6539fe8c4625151e
Merged-In: If840f35245c2d049401d0d2f6539fe8c4625151e
(cherry picked from commit bfbf9608)