Document Signature equals/hash should not include flags

Some of the methods in PackageParser.SigningDetails rely on the
Signature class not including the flags / capabilities when
computing the hash code or performing equality checks. This commit
adds comments to the Signature class to highlight this and adds
tests to detect if this behavior is changed.

Bug: 176814921
Test: atest SignatureTest
Change-Id: I28c591af559568d498449f1c6cbceb1c292d2ad5