Reorganize rollback data storage layout.

In preparation for supporting multi-package rollbacks.

The previous layout had a single directory used for a rollback and a
particular package. For example:

com.package.A-XXX/
    base.apk
    info.json
    enabled.txt

The new layout has separate directories to represent the rollback and
information about a package belonging to the rollback, so it's possible
to have multiple packages in a single rollback. For example:

XXX/
    com.package.A/
        base.apk
        info.json
    com.package.B/
        base.apk
        info.json
    enabled.txt

Where XXX is a random unique string generated by createTempDirectory.

Test: atest RollbackTest
Bug: 112431924
Change-Id: I6cc3b276d6f5f0c6d0852b183dd33c695f6a3fc1