Flag for more parallelization in package parsing

Current behavior when scanning system partitions for apps to install is
to parse packages in a directory and install them before moving on to
the next directory. This flag will gate whether PM is allowed to
continue parsing packages before installs in a directory have completed.

Test: presubmit
Bug: 401622806
Flag: android.content.pm.parallel_package_parsing_across_system_dirs
Change-Id: I9b265c981f0498d5f8f371f8eef919e2b0457572