Store meta data for a rollback in a single file.

Rather than spread it out across multiple per-package info.json files.
This will simplify future changes to the rollback metadata format.

Before this change, the data layout looked like:

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

After this change, the data layout looks like:

 .../
    rollback.json
    com.package.A/
        base.apk
    com.package.B/
        base.apk

Bug: 112431924
Test: atest RollbackTest
Change-Id: I2a29dc6063e02c502eb6f73c56a4590904de8739