Add PLATFORM_SEPOLICY_VERSION.

Create PLATFORM_SEPOLICY_VERSION, which is a version string to represent
the platform sepolicy of the form "NN.m" where "NN" mirrors the
PLATFORM_SDK_VERSION and "m" is a policy-based minor version that is
incremented with every policy change that requires a new backward-compatible
mapping file to be added to allow for future-proofing vendor policy against
future platform policy.

Bug: 36783775
Test: Device boots when sha256 doesn't match and compilation is forced.
Change-Id: I4edb29824f2050a5a6e1bc078c100cf42e45c303