Block authentication and wait for auth thread

Doesn't quite work, deadlock when app is booted without wifi but then
wifi is enabled.
Move delete/unlock waits to AsyncTask
Add a timeout to prevent infinite loop
Safely dereference context in GoogleDriveAccount.startLinkOrAuth()
Use AsyncTask to launch file chooser from VolumeListActivity
Call Account.authIfNeeded() from all async tasks
Add linking to Account.linkOrAuthIfNeeded()
Make Account.linkOrAuthIfNeeded() a member function
Unset linkInProgress after linking in GoogleDriveAccount
Refresh error dialog text in VolumeListActivity.onPrepareDialog()
Comment update in FileChooserActivity.onCreate()
Clear authInProgress in GoogleDriveAccount.unlink()