[s13n] Convert SigninTracker to observing IdentityManager

IdentityManager is implemented on top of SigninManager, PO2TS,
GaiaCookieServiceManager, etc, in the servicified world.

This CL convers SigninTracker away from the aforementioned classes
to use the IdentityManager API.

BUG=893133

Change-Id: Ia1f02a35a3a740b479cabba89ade0ff0ff0e85a5
Reviewed-on: https://chromium-review.googlesource.com/c/1403159
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Cr-Commit-Position: refs/heads/master@{#621551}
4 files changed