Extend backup rules scheme with cross-platform-transfer configuration

The data extraction rules configuration is extended with a new
`cross-platform-transfer` tag that apps can use to opt-in to cross
platform transfers and configure it separately from cloud backup and
regular Android to Android device transfers.

In addition to `include` and `exclude` tags, the new
`cross-platform-transfer` tag should also contain a
`platform-specific-params` tag. This is used to help identify the
corresponding app on the other platform.

Bug: 403956528
Test: atest FullBackupTest.java
Flag: com.android.server.backup.enable_cross_platform_transfer

Change-Id: Ifedb55d835cf4098176b8ddc3d29ea63684f3fb8