apexd_prop is defined for PRODUCT_COMPATIBLE_PROPERTY = false case

apexd_prop is also defined in private/propery_contexts in order to make
the type to exist even when PRODUCT_COMPATIBLE_PROPERTY is false (i.e.,
Pixel 1).

Bug: 119220815
Test: m plat_property_contexts for sailfish
the built plat_property_contexts has apexd_prop defined

Change-Id: I9ef71410533c1f64fc6a3112cfcb199d23aaf3db