vendor: add custom backuptools and postinstall script for A/B OTAs

* A/B OTA devices wont run backuptools in recovery (because they don't
  go in to recovery to do an OTA). In these cases let's use a modified
  version to backup/restore from within android upon postinstall.

* Add backuptool_postinstall.sh which will be run prior to the normal
  postinstall script in order to backup/restore via addon.d scripts.

* This needs to be done in such a manner because we need /postinstall
  mounted rw instead of the ro with context= options which are used for
  the normal postinstall (dexopt) script.

Change-Id: I51511870634dd1ec5388adafddb446f95cc5a950