Fix BackgroundDexOptServiceIntegrationTests

- Ensure that calling uid is shell.
- Clear calling identity such that we don't get permission
failures when calling getCurrentUser()

Test: self
Exempt-From-Owner-Approval: cherry-pick
Merged-In: Ifbaceb47edbbc4a6b002d49411ca4635ffc33a08
Change-Id: Ifbaceb47edbbc4a6b002d49411ca4635ffc33a08
Fixes: 111798412