Load {default,build}.prop from /{system,vendor,odm}/etc/

/default.prop may be moved to /system/etc/ and renamed as prop.default.
/{system,vendor,odm}/build.prop may be moved to /{system,vendor,odm}/etc/
and renamed as prop.build.
This patch makes property_service load prop.* from the new locations
first. Also replaces NULL with nullptr for C++ cleanup.

Bug: 37815285
Test: Booted pixel phones, checked the locations of prop.*, verified
      symlinks, checked a few properties (via adb shell getprop)
      and manually tested a few apps (Camera, Maps etc).
      ODM is not tested as it does not exist on the tested device.
Change-Id: Id8eeb3c1a5728dd04005dfad4733b8e6336f9093