Better Soong namespace support in the RBC runtime.

Includes:
* Defining namespace again should have no effect
* Allow appending to a variable defined in a namespace
* Consistent naming
* Print namespace variable assignments even if they are empty

Bug: 200297238
Test: rbcrun make/build/tests/run.rbc
Change-Id: I64aa22c4498ae89e4bc3a077d6206ad37d5c7c38

tests/run.rbc

@@ -80,7 +80,7 @@ ns = globals["$SOONG_CONFIG_NAMESPACES"]
 assert_eq(
     {
         "NS1": {
-            "v1": "abc",
+            "v1": "abc abc_part1",
             "v2": "def"
         },
         "NS2": {