Don't enforce visiblity on platformBootclasspathImplLibDepTag

When the dependencies that use platformBootclasspathImplLibDepTag
are moved earlier they are checked by the visibility enforcement
mutator.  Implement ExcludeFromVisibilityEnforcementTag so they
don't require visibility.

Test: go test ./...
Change-Id: I6019228561530bd406936fd0d2a16eff471c148d