Bug 969227 - Handle X-Backoff headers.
Bug 958900 - Pre: correct comment.
Bug 958900 - Pre: fix JavaDoc.
Bug 958900 - Part 1: schedule a periodic sync for new FxAccountSyncAdapter.
Bug 958900 - Part 2: refactor out BackoffHandler.
Partially refactor BackoffHandler.shouldSync out of SyncAdapter.
Bug 958900 - Part 3: extract SchedulePolicy out of FxAccountSyncAdapter.
Bug 958900 - Part 4: flesh out backoff and scheduling in FxAccountSyncAdapter.
Move PREF_EARLIEST_NEXT_SYNC into PrefsBackoffHandler, and generalize the suffix to support multiple backoffs.
Rename GlobalSessionCallback.shouldBackoff to shouldBackoffStorage.
Rename SessionCallback's BackoffHandler to storageBackoffHandler.
Add handleBackoff to TokenServerClientDelegate.
Handle Retry-After and X-Backoff in TokenServerClient.
Separate token, storage, and scheduling backoffs.