backuptool: Take into account new location for system default props

 * System default props defined using PRODUCT_SYSTEM_DEFAULT_PROPERTIES
   are stored into /system/etc/prop.default, so that's the location where
   ro.lineage.version prop needs to be checked now. Although, fallback
   to the old location to allow sucessful upgrades.

Change-Id: I62046447876c2198a0c4f88a4f36f4723d417617