Enforce updatable=true on apps of updatable apexes

- Update apex_info (a topdown mutator) so that it sets updatable=true on
  apps of updatable apexes
- Write a unit test that tests different combinations of
  updatable/non-updatable apks-in-apexes
- Update an existing unit test that asserts a different error

Test: go test ./java
Test: m nothing (in internal)
Bug: 209409604

Change-Id: Ie8881b857afcec44addf27fc360c5b8abf726bd2