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Regular USS datatypes (bridges) don't need to care about encryption,
because the processor and worker take care and the original model
doesn't need to be encrypted.

PASSWORDS on pseudo-USS are special because we need that the "directory"
copy (the one in SyncableServiceBasedBridge) is stored encrypted on
disk too, similarly to how the legacy Directory does it.

In certain cases like when setting up a custom passphrase, the
encryption key changes and the processor&worker react to that by
recommitting all entities with the new encryption requirements. For
pseudo-USS PASSWORDS, it should also reencrypt the local "directory".

In order to do this:
1. A new method is introduced in ModelTypeSyncBridge, for the bridge
   to realize there's an ongoing reencryption.

2. SyncableServiceBasedBridge takes care of using the cryptographer
   (available for PASSWORDS only) to reencrypt all data.

Implementation-wise, the simplest way to achieve that is to modify
the bridge such that in_memory_store_ keeps more information, namely
the whole sync_pb::PersistedEntityData proto.

Bug: 870624
Change-Id: I1e0d7c972580377618c05b9d1f79c6d72f58022f
Reviewed-on: https://chromium-review.googlesource.com/c/1288629
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601863}