Avoid loading duplicate jobs.

If the persisted files have somehow accumulated duplicate versions of
the same job (identified by uid-namespace-jobId), then drop the
duplicates and only keep one.

Bug: 289062813
Bug: 296890885
Test: atest JobStoreTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c8fbd937940b83944492c4ccaf2830611285999a)
Merged-In: I3179261309031801f0506cf905206376e59af1fe
Change-Id: I3179261309031801f0506cf905206376e59af1fe