Rename onMeasureFullBackup to onEstimateFullBackupBytes

And annotate the method so it's clear what kind of value it's returning.

Bug: 430332837
Flag: com.android.server.backup.enable_cross_platform_transfer
Test: atest core/tests/coretests/src/android/app/backup/BackupAgentTest.java
Test: atest services/robotests/backup/src/android/app/backup/ForwardingBackupAgent.java
Change-Id: Ib27e1c980135e2bd34e4a33f45f459a04d2fdcbe