Use session id to search for rollbacks (2/n)

1. #enableRollback is greatly simplified since now we can get session
   objects directly using the session id.
2. ACTION_CANCEL_ENABLE_ROLLBACK handler now uses the session id to
   search for rollbacks to delete. Since 'token' is not used, we will be
   able to remove unused code and APIs later.

(Cherry-picked from 0a5a4db2)

Bug: 149663536
Test: atest RollbackTest
Merged-In: I2d0bc2166348ff7b5cdee7ac0b31a61023e33cb8
Change-Id: I2d0bc2166348ff7b5cdee7ac0b31a61023e33cb8