selinux: use the policy version defined in sepolicy

In the current setup, init uses the highest policy version supported
by the kernel, instead of the policy version defined in policy. This
results in inconsistency between precompiled (version 30) and
on-device compiled policy (version 30 or 31). Make these consistent.

Bug: 124499219
Test: build and boot a device. Try both precompiled and on-device
compiled policy.

Change-Id: I0ce181916f43db17244c4d80f5cf5a91bbb58d3a