checkapi: enum type for new field should have 0

checkapi checks if new fields has a default value or it is nullable. But
enum type can't have a default value(b/142893595) and can't be nullable.

So checkapi should allow enum types as a new field if enum types
have 0 as a valid value (enum type's implicit default value).

Bug: 171003934
Test: aidl_unittests / aidl_integeration_test
Change-Id: Ie4f262121e623cfe983111beb672a311b2a23128
2 files changed