Change mapping file name to reflect its platform version.

As the platform progresses in the split SELinux world, the platform
will need to maintain mapping files back to previous platform versions
to maintain backwards compatibility with vendor images which have SELinux
policy written based on the older versions.  This requires shipping multiple
mapping files with the system image so that the right one can be selected.
Change the name and location of the mapping file to reflect this.  Also add
a file to the vendor partition indicating which version is being targeted that
the platform can use to determine which mapping file to choose.

Bug: 36783775
Test: Force compilation of sepolicy on-device with mapping file changed
to new location and name, using the value reported on /vendor.

Change-Id: I93ab3e52c2c80c493719dc3825bc731867ea76d4