Standby exemption for system gallery's backup jobs

New media requiring backup can appear on the device without the user
directly using the gallery app. If the gallery app is responsible for
media backups, this can lead to a lot of pending backup jobs because of
the likely low-priority standby bucket of the gallery app.
We should be lenient in imposing throttling to such jobs.

Test: atest FrameworksMockingServicesTests:JobStatusTest

Bug: 148887841
Change-Id: I54a42037c335f86a9078cf15e3e8d745d03c7337