Ensure that DepIsInSameApex is not called for ExcludeFromApexContentsTag

The ExcludeFromApexContentsTag marker interface was added to avoid
every implementation of DepIsInSameApex() from having to deal with the
special tags, like PrebuiltDepTag. Unfortunately, when adding that
not all calls to DepIsInSameApex() were protected which meant that the
BootImageModule, which panics if it doesn't recognize a tag, was
causing failures. This change documents the need and improves the
consistency.

A follow up change will add a test for this.

Bug: 182992071
Test: m nothing
Change-Id: If0bf9a7447ebf7a0bb0c88e91951a7220d4af45c