refactor(client): Remove global variables in fxa-client.js

* Replace the global variables in fxa-client.js with instance variables.
* All view tests create their own fxa-client to avoid cross test pollution.
* One fxa-client instance is shared across views for the application.

Work done as part of the evaluation of the amount of work to replace fxa-js-client with a mock for testing. (See #1356)

fixes #1304
Merge pull request #1363 from mozilla/issue-1304-no-globals-fxa-client

refactor(client): Remove global variables in fxa-client.js