CodeOwnerResolverResult: Include flag if there are unresolved code owners

Android wants a configuration option to make all users code owners of
paths for which no code owners are defined:

1. if for a path no code owners are defined, the path is owned by all
   users
2. if for a path code owners are defined, only these users own the path
3. if for a path code owners are defined, but none of them is resolvable
   the path is owned by no one and changes require an override for
   submission

As a first step for adding such a configuration option we extend
CodeOwnerResolverResult with a flag that is set if there are unresolved
code owners. This allows callers to differentiate the cases above:

1. codeOwners is empty, ownedByAllUsers = false,
   hasUnresolvedCodeOwners = false
2. codeOwner is non-empty or ownedByAllUsers = true
3. codeOwners is empty, ownedByAllUsers = false,
   hasUnresolvedCodeOwners = true

Signed-off-by: Edwin Kempin <ekempin@google.com>
Change-Id: Iab1abf7a139dcead9e8ccad55b989a00d53ff8fc