Log cancellation reason for full backups to BMM

Currently it's not possible to know from these logs if the backup was
cancelled due to timeout or the app crashing or due to backups being
explicitly cancelled by an external caller. This CL fixes that issue.

Flag: EXEMPT log only update
Test: manual // PFTBT is notoriously impossible to write unit tests for.
Bug: 399645990
Change-Id: Ia18fdec1d9547130926bf62666ac055e320fb0b6