[FSD2D] Add migration-aware methods to AppBackupUtils

This CL will be part of a series to implement backup / restore of all
app data during a device-to-device migration. Detailed overview of the changes at go/br-fsd2d-design-doc-app-data.

Bug: 160407842
Test: atest AppBackupUtilsTest
Change-Id: I35ce627f1d372437d8ba8495029c6df15db31552