Only maintain maps between current and previous selinux versions.

New maintenance scheme for mapping files:
Say, V is the current SELinux platform version, then at any point in time we
only maintain (V->V-1) mapping. (V->V-n) map is constructed from top (V->V-n+1)
and bottom (V-n+1->V-n) without changes to previously maintained mapping files.

Caveats:
- 26.0.cil doesn't technically represent 27.0->26.0 map, but rather
current->26.0. We'll fully migrate to the scheme with future releases.

Bug: 67510052
Test: adding new public type only requires changing the latest compat map
Change-Id: Iab5564e887ef2c8004cb493505dd56c6220c61f8