Load build sysprops early

*/build.prop files are now loaded much earlier than before; from 'on
post-fs' to the time when the property service is started which is
before init starts the action loop.

This ensures that all processes that are launched by init have a
consistent view of system properties. Previously, the processes that
started before 'on post-fs' were initially with the small number of
sysprops loaded from */default.prop and then suddenly get additional
sysprops from */build.prop while they are executing.

Bug: 122714998
Test: device boots

Change-Id: Ic07528421dfbe8d4f43673cea41175d33cfbf298