* 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
