Ensure CancelUpdate() always works in recovery.

This adds a new helper function, TryCancelUpdate(), and refactors the
update cancellation logic. It now takes a "force" parameter, which if is
set and the device is in recovery, overrides all conditions that would
cause cancellation to fail.

In addition the cancellation logic has been consolidated to avoid
duplication of code and error checks. Removal of invalid snapshots is
now automatic for all success cases.

Test: vts_libsnapshot_test
Bug: 352538505
Change-Id: I612d4eb3c4f6436838c9b2f1e38bf644e99c6ad0