Add reverse-attribute mapping to sepolicy-analyze.

sepolicy-analyze allows users to see all types that have a given
attribute, but not the reverse case: all attributes of a given type.
Add a '--reverse' option which enables this, but keeps the previous
interface.

Usage: sepolicy-analyze sepolicy attribute -r init

Bug: 36508258
Test: Build and run against current policy.

(cherry picked from commit d444ebed)

Change-Id: I9813ebf61d50fb5abbc8e52be4cf62751979bbd4