Fix mapping file build.

Location of mapping files has changed from private/mapping/V.v.cil to
private/compat/V.v/V.v.cil
Change the build rule for current_mapping.cil to reflect that.

Test: Build current mapping file with  BOARD_SEPOLICY_VERS := 27.0 and
make sure that $OUT/obj/ETC/27.0.cil_intermediates/27.0.cil is not empty
Change-Id: I996a717e1c659265cb067da5d621d71ff3b3b63b