Add an api for platform sepolicy version to vintf

PLATFORM_SEPOLICY_VERSION will be the latest version in system's
compatibility matrix. This change adds a function
getPlatformSepolicyVersion, which ultimately gets
PLATFORM_SEPOLICY_VERSION, just like getSepolicyVersion for vendor.

This will be used from the SELinux CTS.

Bug: 186596569
Test: atest CtsSecurityHostTestCases:android.security.cts.SELinuxNeverallowRulesTest
Change-Id: Ic7863ece9413b9a6a4a91ff42d3121935823fe8b