JarFile: make getManifestFromReference and getManEntry synchronized

So that all modifications to the cached manifest entry and to the
underlying JarVerifier are synchronized.

computedHasClassPathAttribute and hasClassPathAttribute are just cached
booleans that don't change once set, unsynchronized access to them would
cause only for them to be computed twice in the worst case.

Also, removed dead code and corresponding test.

Bug: 27826114
Change-Id: Id3a9b74997cb462c625861c0284196f74dcb8e9a