[Signin][Android] Add SigninActivityMonitor

SigninActivityMonitor tracks sign-in activities started from SigninUtils
and exposes this as an observable value. This CL also wires up
ConsistencyCookieManager so it tracks this value and exposes it for the
native side.

Bug: 949562
Change-Id: If2e5f9b596bd3d03a8d73d3606785e1aff020300
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1578548
Commit-Queue: David Roger <droger@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#653509}
4 files changed