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.
Change-Id: Ice6893cf7aa2ec4706a7411645a8e0a8a3ad01eb