partitionmanager: support delayed adopted storage mount

* On some devices, the external mmc driver will only load the card after
  a certain delay. The original code will break and fail to mount any
  adopted storage in this case.

* Add a special fstab flag to allow the partitionmanager to retry after
  a delay.

Change-Id: I219978850ff3ba7ad9dc7ca96fc30634c9574164
3 files changed