Get OperationType from transport

Earlier we introduced versions of BackupManager#requestBackup() and
BackupManager#beginRestoreSession() that take @OperationType as a
parameter (i.e. backup or device-to-device migration). Change the logic
to determine the operation type from the properties of the transport
(BackupTransport#getTransportFlags()). This will help avoid introducing
unnecessary APIs and save us from having to ensure consitency between
@OperationType passed through to BackupManager and properties of the
transport used for  that operation.

Bug: 174216309
Test: atest UserBackupManagerServiceTest
Change-Id: I04a6014087c98efdeb8710fadc620b0856d7554b