RecoverySnapshotStorage: fold readFromDisk() and writeFromDisk() into callers

readFromDisk() and writeFromDisk() each have only one caller, get() and
put() respectively.  Fold them into these callers to make the code more
straightforward.  No change in behavior.

Flag: EXEMPT refactor
Bug: 372499066
Test: atest FrameworksServicesTests:com.android.server.locksettings.recoverablekeystore
Change-Id: I11aa7bac563fe9c87bcf3d17cec02db5ead869c9