Also fix up set() additions in mini_parser.py and add global reference to
the parser in tests for clarity.

Bug: 36899958
Test: rm public type in old policy from policy and observe test failure.
Change-Id: I6cba2473526798be871cd69249c9bbc6df2c5b4c

Add support to the treble_sepolicy_tests suite that explicitly look at
the old and current policy versions, as well as the compatibility file,
to determine if any new types have been added without a compatibility
entry.  This first test catches the most common and likely changes that
could change the type label of an object for which vendor policy may have
needed access.  It also should prove the basis for additional compatibility
checks between old and new policies.

Bug: 36899958
Test: Policy builds and tests pass.
Change-Id: I609c913e6354eb10a04cc1a029ddd9fa0e592a4c