installd: Add support for migrating legacy obb data.

Data needs to be copied over from /data/media/obb to its
new location under /data/media/0/Android/obb.

This is currently implemented as a single "cp -r" between
the two paths. The cp is executed by a forked shell script
that runs in its own domain.

Test: Manua
Bug: 129167772

Change-Id: I8a0667f33f7fcee5c9e0dba7b00a963797b0dc34