Fix that backupFinished() callback is not called sometimes.

Before this in case of TRANSPORT_ERROR backup pass was aborted before backupFinished() call.
Now this happens in 'finally' block so that there is no way to avoid it.
Also, now backup pass doesn't break in case of QUOTA_EXCEEDED result for single package.
And some refactoring around 'currentPackage' variable.

Bug: 27094847
Change-Id: I18df3f500b427381f32bd11ed1aa87ab9577bc91