Propagate class @Immutable.Ignore to references

If a class is marked at its header with @Immutable.Ignore, the
processor should ignore all usages and references to the class,
regardless of it's used as a return type or type arg.

Without this change, it was required to mark @Ignore on all of
usages of the class.

Test: atest android.processor.ImmutabilityProcessorTest#ignoredClass

Change-Id: I5cd470d86c6eb8c3358656a6d8f6e2588bdb6c66