Remove some globals from system_properties

pa_size should be static to prop_area, so make it so.

__system_property_area__ was reused for various purposes, but
realistically is a deprecated symbol and this finally separates us
from it.

Bug: 36001741
Test: boot bullhead, system property unit tests
Change-Id: I39663cc3b613093fa4c728b21d8ba58754f8e105