Export dummy functions for libselinux_vendor

The following functions are exported in libselinux_vendor, but
simply returns an error:

selinux_android_prop_context_handle
selinux_android_restorecon

Bug: 62886649
Test: BOARD_VNDK_VERSION=current m libselinux_vendor -j
Test: m libselinux_vendor -j
Test: device boots
Change-Id: If42b6cb1bf9ffb3dd04f557279efdc37e1ac259a
Merged-In: If42b6cb1bf9ffb3dd04f557279efdc37e1ac259a
(cherry picked from commit 1e8760189dc12a7ff7e40f97da4100e71ceac123)
5 files changed