Ignore PrebuiltDepTag when processing APEX contents

When a source and a prebuilt module are present in the same build a
dependency is added from the source module to the prebuilt module.
Previously, the code for generating the APEX did not recognize that
tag and in some cases (e.g. for cc_(prebuilt_)library_shared) will
fail the build.

This change:
1) Adds a test to reproduce the problem.
2) Improves the debug message by pretty printing the tag.
3) Adds a new ExcludeFromApexContents interface that can be implemented
   by a tag to declare that it should be excluded from the APEX
   contents.
4) Ignores tags that implement that interface when generating APEX
   contents.
5) Implements that interface on prebuiltDependencyTag to fix the
   test.

Bug: 153326844
Test: m nothing
Change-Id: I9dd4312c4f995c816c0a31d8d733eb5d7f56e1ea