Refactor package_outputs

Refactor package_outputs in the TestOptimizer so it just returns a list
of soong_zip commands to be run by build_test_suites.

Since we already have a tested implementation for running subprocesses
in build_test_suites.py there's no reason to reimplement it in
optimized_targets.py. Because any packaging will ultimately use
soong_zip to package its final outputs change the code to just do
whatever prep it needs to and return a list of soong_zip commands.

This way the code is simpler to test without requiring subprocesses and
no reimplementation of subprocess running code is necessary.

Test: atest build_test_suites_test; atest optimized_targets_test
Bug: 358215235
Change-Id: I3025aefeeb7186f537266a72d8422211ca9835ba