Better iteration over mCurrentOperations in cancelBackups

handleCancel was being called on an operation
while we are iterating over all operations.
This can cause issues if handleCancel modifies
the array containing all operations.

Test: Run cancelBackup GTS tests.

Change-Id: If3fbe7b5d9e61aa870f8f6e0dd1654e735c0c5b8
1 file changed