Fix SignatureBuilder.

It seems the compiler has started adding a top level MODULE which broke
assumptions made in here.

Also add some tests to verify this and other basic functionality, so that
future such breaks may be caught sooner.

Test: m framework-annotation-proc-index
Test: atest unsupportedappusage-processor-test
Bug: 144495446
Change-Id: I43efb25c1600af15809d48aa84498a6800f713b1