System Health: Support expiring tokens

In the past android:customTokens=true authenticators were required to handle
their own token caching. This is detrimental for battery when high traffic
authenticators are constantly spinning up processes to start services to do
file io to check their own caches.  This change allows authenticator
implementers to optionally let the framework do some of the work for them by
providing the framework with a expiration time.

The AccountManagerService will make a best effort to re-use the cached
token if possible.

Bug: 21530782

Change-Id: I16a7edba36a220e3891e55cf61c725c2be863323