java/syncbase: Add LoginFragment for Android login

Android login is easier than ever with this wrapper!

The LoginFragment will attach a fragment to the given activity. Then,
- It uses the AccountManager to ask the user to pick an account.
- Once selected, it retrieves the corresponding OAuth token.
- Once that is retrieved, it calls login with the corresponding
  callback calls.

Change-Id: I00383b60628e085ff702f76d8342003c3f6b8148
3 files changed