Refine art/build/apex/runtests.sh's pretty printing options.

- Rename option `-l, --list-files` as `-t, --print-tree` to better
  reflect its action.
- Make option `-t, --print-tree` also display file sizes.
- Only check for the presence of `tree` if option `-t, --print-tree`
  is used.
- Introduce a new option `-l, --list-files`, using `find` to display
  the image's contents.
- Refactor this logic into a function (`maybe_list_apex_contents`),
  so that it can be reused for all APEX packages.

Test: art/build/apex/runtests.sh -t
Test: art/build/apex/runtests.sh -l
Bug: 113373927
Change-Id: I31fefe0161dc4e4d166baa76bb5e6611bf5940dc