This property is set when HAVE_SELINUX:=true during build.

This will provide a convenient method for determing build
configuration when in the Java layer. For instance, this will
be used in change id I69529fb7a3adfe31eccb16d79740fc4952ff5e68
to determine if the Settings app should display the SE Linux
status.

Change-Id: Idc8dff020aaac41649eac64f5fa8e311dabbc51f