Import OEM build properties, if defined.

Certain products can define a list of system properties that should be
delegated to the OEM.  Since these properties may be ro.*, we give
them first shot at defining.

Also support blacklist of properties that should never be defined by
build.prop, used to delegate to runtime fingerprint generation.

Bug: 13367676
Change-Id: I3f00db732f485818205a7b76b31edbdc3a89aac0