Add `onMeasureFullBackup` method to `BackupAgent`

When backing up, a preflight check is performed that checks the size of
the backup against the backup quota. By default, the backup agent
measures the size of a backup by performing a full backup and discarding
the backed up data. This means for a normal backup session, the
`BackupAgent#onFullBackup` is called twice. For most backup agents this
is fine, as they won't have much data to back up.

We want to give apps that back up a lot of data (for example, during D2D
where the backup quota is multiple gigabytes) the possibility to provide
their own implementation of measuring the backup size so they can
provide a more efficient implementation if needed.

Bug: 403956528
Test: atest BackupAgentTest.java
Test: atest MeasureFullBackupTest.java (cts test added in same topic)
Flag: com.android.server.backup.enable_cross_platform_transfer
Change-Id: If6a869e772d433acc770f47f4aa05f54c54d3c0e