Revert "Revert "Load default prop from /system/etc/prop.default""

This reverts commit 98a73a2c.

Bug: 37815285
Bug: 62525809
Test: Tested with ag/2398663 and ag/2400524. Booted pixel phones,
      checked the location of pro.default, verified the symlink
      at /default.prop, checked a few properties via adb shell and
      manually tested a few apps.
      Booted to recovery mode and ran 'adb sideload' successfully.
Change-Id: I407412a7002b898ffb352cb5f331cab9c15be39a