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- Add code to add/remove/get cryptohome keys for Easy sign-in;
- Add an EasyUnlockKeyManager to wrap the operations and provide an API;
- Re-create crypthome keys with pairing data in user prefs on user sign-in;

BUG=394640,394641

Review URL: https://codereview.chromium.org/554043003

Cr-Commit-Position: refs/heads/master@{#294901}