update_engine: Run backuptool script before normal postinstall script

* Check if the currently running /system has ever been mounted R/W.
  If it has, we can assume that running backuptool_ab scripts won't
  do anything catastrophic like break verity. Also this is a good
  indicator that the partition has been messed with since the initial
  installation of the rom (for instance if a gapps or su package had
  been added).

* Mount the new target slot's /system partition at /postinstall R/W,
  before the normal dex preopt script is run, and perform backuptool_ab
  operations. This will allow retaining addon packages, similar to what
  we've historically used in recovery flashes for non-A/B devices. This,
  since it mounts the new /system R/W, will also ensure it gets triggered
  again on the next flash.

* This assume that all backuptool operations are contained in /system
  and that we won't ever screw around with retaining things on /vendor.

Change-Id: I00ca479ccc24080bf3457334a6c9e4c07dd20f45

0 files changed, 0 insertions, 0 deletions