init: use platform sepolicy version indicated by /vendor.

It's possible, in the event of a platform update, for the platform
SELinux policy to change from the policy on which the vendor SELinux
policy was originally based.  In this case, a different mapping file
to bridge the differences between the new policy and the old needs to
be selected.

Make init choose which mapping policy file to use based on the version
reported in /vendor/etc/selinux/plat_sepolicy_vers.txt.

Bug: 36783775
Test: Force compilation of sepolicy on-device with mapping file changed
to new location and name, using the value reported on /vendor.

Change-Id: I63c883ccb79dd31c92dabe44a55c4ab50a3735e6