Move mapping_sepolicy.cil to /system partition.

This is a necessary first step to finalizing the SELinux policy build
process.  The mapping_sepolicy.cil file is required to provide backward
compatibility with the indicated vendor-targeted version.

This still needs to be extended to provide N mapping files and corresponding
SHA256 outputs, one for each of the N previous platform versions with which
we're backward-compatible.

Bug: 36783775
Test: boot device with matching sha256 and non-matching and verify that
device boots and uses either precompiled or compiled policy as needed. Also
verify that mapping_sepolicy.cil has moved.

Change-Id: I5692fb87c7ec0f3ae9ca611f76847ccff9182375