Add a PersistentInt class.

This implements a simple integer written to disk backed by
AtomicFile.

Bug: 230289468
Test: new unit test
Change-Id: I1c73c8ac2429f92153cfba3b006b4374e75e65df
Merged-In: I1c73c8ac2429f92153cfba3b006b4374e75e65df
  (pure cherry-picked from ag/18437645)
2 files changed