[Windows] Rework Android NDKs installation (#2555)

* Rework Android NDKs

* Minor fix

* Fix variable name

* Minor fixes

* Reword Android env vars table

* Remove Android env vars table from doc file

* Rework to use only major ndk versions in toolset

* Fix ndk paths

* Fix win2016 toolset

* Refactor

* Minor fix

* Fix ndk tests

images/win/toolsets/toolset-2016.json

@@ -163,9 +163,12 @@
             "cmake;3.6.4111459",
             "cmake;3.10.2.4988404",
             "patcher;v4",
-            "ndk;21.3.6528147",
             "cmdline-tools;latest"
-        ]
+        ],
+        "ndk": {
+            "lts": "21",
+            "latest": "22"
+        }
     },
     "visualStudio": {
         "version" : "2017",