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This makes EasyUnlockService pure virtual base class and introduces
separate implementations for regular and signin profiles. Common code
for both service types is kept in EasyUnlockService class.

EasyunlockService for signin profile is left empty.

BUG=401634

Review URL: https://codereview.chromium.org/577683002

Cr-Commit-Position: refs/heads/master@{#295188}