Create platform config parser and use it to check backup eligibility

When the transport is doing a cross-platform transfer, we only allow
apps that have explicitly opted-in to cross-platform transfer via their
data extraction rules to back up. The only exception is the `@pm@`
key-value backup because that is required for all backups.

While the data extraction rules are a good place to have cross-platform
specific configuration, this was originally only used by the full backup
implementation in the backup agent. Ideally we would re-use the existing
parsing code in FullBackup.BackupScheme, however this class currently
assumes it is running in the context of the application being backed up.
The eligibility checks are done in the system server, so we can't
directly use the existing BackupScheme class.

This is why I've re-implemented part of the parsing in a separate class,
which is isolated to the code that only runs as part of cross-platform
transfer. Ideally we would remove the dependency on the application
context from BackupScheme and combine the parsers in the future, I've
filed b/439533655 to track this work.

Note: the code in this change only triggers when the backup transport is
doing a cross-platform transfer, this is a new API that is flagged.
Existing transports do not yet set this transport flag, but this will be
rolled out behind their own feature flags.

Flag: com.android.server.backup.enable_cross_platform_transfer
Bug: 432673356
Test: atest BackupEligibilityRulesTest
Test: atest PlatformConfigParserTest
Test: manual testing

Change-Id: I419a492a677d3098d7704805a6e85550df140bb6