Add .packages() to Nene.

The API is similar to .users() - Every method relating to packages must
specify the user it applies to. I also experimented with an alternative
api which looks like `testApis.forUser(user).packages()....` but it
becomes a lot more complex, as we need to maintain a set of APIs which
apply to all users and one which applies to individual users.

Bug: 180293870
Test: atest NeneTest
Change-Id: I61515f8bd0299fdd2459f97f5018b77cd97fb651
27 files changed