init: Use sepolicy version instead

This commit uses vendor sepolicy file version (defined in
`/vendor/etc/selinux/plat_sepolicy_vers.txt`) to determine whether the
source context should be set as `u:r:vendor_init:s0`.

Before this commit, the criterion was `ro.vndk.version` >= 28.  However,
the check in `property_service.cpp` will always be true because
`ro.vndk.version` hasn't been loaded from `/vendor/default.prop`.

Furthermore, under some circumstances, `ro.vndk.version` may be
different from `plat_sepolicy_vers.txt` (e.g. O-MR1 vendor does not
define `ro.vndk.version`).

Bug: 78605339  # high-level bug to combine O-MR1 and P GSI
Bug: 79135481  # the usage of `ro.vndk.version` in init
Test: vts-tradefed run vts -m VtsTrebleVintfTest  # tetheroffload
Change-Id: Ied46e9346b4ca7931aa4dcf1c9dbc11de0e12d93