Pass in cancellation reason to B&R tasks

This is to give more granular information to tasks about why they got
cancelled. This will help them report why they failed.

The cancelAll parameter was only set to false for cancellations due to
timeouts, so I've removed it.

This is a no-op refactor.

Flag: EXEMPT refactor
Bug: 399645990
Test: atest CtsBackupTestCases
Change-Id: I1bd6d9eff42c5edeb0f4df453bae792cf994bb6a