Stop backing up the ancestral record version unless it has changed

The ancestral record version is a number that's contained in the backup
created by the PackageManagerBackupAgent. It's used to indicate how to
restore the backed up data. This allows the PackageManagerBackupAgent to
change how it stores its data without worrying that it may break other
versions of the backup agent during restore.

Backup agents should only back up key/value pairs that haven't changed
since the last time a backup was done. It should do this by maintaining
a local state. The agent is given the state file from the previous
backup run (if any) and is given a file to write the new state to.

The ancestral record version that the PackageManagerBackupAgent writes
to the backup almost never changes. However, this key/value pair was not
stored in the local state. As a result, the backup agent backs up this
key/value pair on every backup causing unnecessary backup traffic.

This change fixes that by storing the ancestral record version in the
local state. It then avoids backing it up again when it's present in the
local state.

It's possible for the ancestral record version in the state file to be
different from the current value in the backup agent, for example after
a system update. If this happens that means a change was made to the
format of the data we back up, so we intentionally back up all key/value
pairs to ensure they're all written in the new format. In addition,
there may be key/value pairs that were backed up in the past but are no
longer backed up (e.g. an app was uninstalled). We remove these
key/value pairs since they were created with the older ancestral record
version but we can't update them.

Bug: 268341874
Test: atest PackageManagerBackupAgentTest.java
Change-Id: Iaa805198801b38b0c4867536749095fff243e18e