Intern strings loaded from persisted file.

Intern the strings loaded and kept from the persisted file so we don't
retain unnecessary duplicates.

Bug: 273758274
Bug: 247127880
Test: atest FrameworksMockingServicesTests:ScribeTest
Test: atest FrameworksServicesTests:JobStoreTest
Change-Id: Id0c9448483cec65aa6f2d30a3295b1300e02c830