fix: --checkapi works with enum constants

--checkapi should compare const values with "literal" values, not
with evaluated values because --checkapi task can't resolve imported
types.

--checkapi=equal was relying on "Dump()" which relies on type/value/
resolution.

Now --checkapi=equal dumps const values using literals, not evaluated
values.

Bug: 179950161
Test: m aidl_unittests
Change-Id: I62c84f060036260bfcd9c1020e32715993378e26
3 files changed