Fix AAA not placed as frontline unit

- Spawn AAA like SHORAD
- change SA-19 unit class from AAA to SHORAD

fixes #2194

resources/units/ground_units/2S6 Tunguska.yaml

@@ -1,4 +1,4 @@
-class: AAA
+class: SHORAD
 description: "The 2K22 Tunguska (Russian: 2\u041A22 '\u0422\u0443\u043D\u0433\u0443\
   \u0441\u043A\u0430') is a Russian tracked self-propelled anti-aircraft weapon armed\
   \ with a surface-to-air gun and missile system. It is designed to provide day and\