Use new bitwise operators for UpdateAttemptFlags

Switch the bitwise operator overloads to the new standardized ones from
libbrillo.

Test: unit-tests
Test: manual exercising of update_engine via APIs
Change-Id: Ia8bd7d90adc9469826a906c127c87bfac8272f6c
(cherry picked from commit 931b955fc5c8427b90a123859e1dab22baae7f7d)