Componentize FakeSigninManager and SigninManager prefs

This CL moves FakeSigninManager to the signin component.
It also moves the registration of SigninManager preferences.

BUG=332107
TBR=jochen

Review URL: https://codereview.chromium.org/1257623002

Cr-Commit-Position: refs/heads/master@{#340311}
31 files changed