Fix default value of BoardSepolicyVers

PlatformSepolicyVersion and BoardSepolicyVers haven't assigned correctly
so far. Below is the reason why it hasn't been discovered yet.

DeviceConfig.PlatformSepolicyVersion() was added to support mixed
sepolicy build (setting BOARD_SEPOLICY_VERS and building vendor sepolicy
with old plat policy files). Soong compares PlatformSepolicyVersion()
and BoardSepolicyVers(), and used old vendor sources if both are
different. Back then, the only place where such logic played a role was
selinux contexts. Test codes were running as intended: after setting
BOARD_SEPOLICY_VERS the context files were built against old policies.

But there were two mistakes:

1) PlatformSepolicyVersion() was not added to soong_config.mk, so it was
always empty.
2) BOARD_SEPOLICY_VERS was set to default in system/sepolicy/Android.mk,
which was processed after soong_config. So if BOARD_SEPOLICY_VERS was
not set in BoardConfig.mk, BoardSepolicyVers() was empty, not
PLATFORM_SEPOLICY_VERSION.

And there were no issues as Soong only checked equality.

To fix the issue correctly, this commit adds the variable
PlatformSepolicyVersion, and then handles BoardSepolicyVers correctly by
returning PlatformSepolicyVersion if it's empty.

Test: set BOARD_SEPOLICY_VERS and see vendor_file_contexts changes
Change-Id: Ie55827f47d2903e9739eccacc3874d354d5c9f2d