Security of Android cryptography in versions < 5.0 against government agencies
What is the real Android security level when using cryptography?


The cryptography is strong itself. 
Its application can be weak in systems of many applications and heterogenous hardware. 
Many outdated applications can also increase the risk of security tweaks. 
I can say that the security level of Google Pixel (sorry Android 7.x) can be strong because of both hardware and software control in the development of the product. 


  Does Google have ways to unlock or decrypt the device without user key? (Maybe, because you can unlock your phone with your Google account, and Google has technical control over your account)


There are no standard ways to do this. 
Secondary ways may exists but they are because of other reasons, than Google.


  Is the widely-announced feature a revising of the classic sdcard protection or a brand new way to encrypt the device?


I think no. I see no revises on the classic sdcard protection system here.