STITCH-2509 fix some logic issues with multi-user authentication

[adamchel]

This should bring the Android multi-user logic to parity with the JS PR logic in mongodb/stitch-js-sdk#216.

Fixes made here include the following:

• various internal naming changes to be consistent with Android
• new doAuthenticatedRequest overload to do authenticated requests on behalf of another user to prevent many prepareAuthRequest calls
• adding USER_NOT_FOUND and USER_NOT_LOGGED_IN error codes
• storing the user list as an ordered map (LinkedHashMap) rather than a LinkedList
• persisting device id when there is no active user
• removing users when not logged in
• removing anonymous users when they are logged out
• handling user linking profile failure edge case
• various changes to the order of logic in CoreStitchAuth
• various test changes

[coveralls]

Coverage remained the same at 67.242% when pulling ae9f3a0 on adamchel:STITCH-2509 into 9cc7916 on mongodb:master.

[tkaye407]

LGTM