Don't call BackupTransport#checkFullBackupSize when preflight timeouted.

mResult in SinglePackageBackupPreflight could be set
to a negative value if preflight timeouted.
Together with ag/863259 this change will better handle this case.

Bug: 26818914
Change-Id: I171bf95f146552b3b50f044964c2b041f6303d90
(cherry picked from commit a235f7e4)