Call removeMessages() only for certain operations in BackupManagerService#handleCancel()

Only remove messages of types OP_TYPE_BACKUP_WAIT and OP_TYPE_RESTORE_WAIT, since OP_TYPE_BACKUP cannot time out and doesn't require cancellation.

This will prevent some unncecessary (false) warnings in logcat.

Bug: 36570881
Test: manual
Change-Id: I72ecf98438fff18616354b04638b7db5c943aa61