Move split sepolicy to correct locations

This moves the CIL files comprising the split sepolicy to the
directories/partitions based on whether the file is part of
platform/system or non-platform/vendor. In particular:
* plat_sepolicy.cil is moved to /system/etc/selinux,
* nonplat_sepolicy.cil is moved to /vendor/etc/selinux, and
* mapping_sepolicy.cil is moved to /vendor/etc/selinux.

Test: Device boots, no additional denials. The test is performed both
      for a device without the CIL files and with the three CIL files.
Bug: 31363362

Change-Id: Ia760d7eb32c80ba72f6409da75d99eb5aae71cd9