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

/default.prop is moved to /system/etc/prop.default for full Treblized
devices. Modifies property_service to load it from there first.

Bug: 37815285
Test: Tested with ag/2371424. 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.

Change-Id: I485231f21fc86b0aec58edf867e229a31e77d85e