More layoutlib testing infra fixes

- Remove ModuleClassLoader as it can be replaced with a URLClassLoader
  for now.
- Move CustomCalendar and CustomDate to a separate package that can be
  used both by the Bridge tests and the actual test app.
- Move empty.xml out of the test app so it compiles.
- Update test app to use the latest build tools and SDK (some attributes
  being used by the app weren't supported in API 21).
- Update gitignore to remove the new out directory.

Test: Update to existing tests
Change-Id: Ieb7324d5ae559f9c581771c57f2127cd83909015