Reintroduce the contract for random token to be non-negative

Clients like adb backup/restore confimation activity
depended on it.

Test: gts-tradefed run gts -m GtsBackupHostTestCases
Bug: 64042228
Bug: 63691912
Bug: 63695367
Change-Id: I09e348f7570bcdbad2d91e79f90a248b17c82940