Fix dropbox login loop

This should fix the issues some users are having with not being able to log on. We were setting a flag in the LoginScreen activity to determine whether to process the authentication result from Dropbox. If this flag was not set, we would skip the authentication logic and just display the login screen. The problem is that the flag is just a variable and would be cleared if the Activity was destroyed. I was able to reproduce this by switching to another app for a few seconds before clicking Allow in the dropbox screen.

It seems, however, that the flag is not necessary. The authentication result logic can be run every time the LoginScreen activity is shown, with no side effects if we haven't yet initiated the Dropbox authentication process. I removed the flag and everything seems to work.