Support complicated variable references

Bug: 226974242
Test: go test
Change-Id: Iaec16f5c498e7c75c9ee5d53d3499efadfba16bc
diff --git a/androidmk/parser/make_strings_test.go b/androidmk/parser/make_strings_test.go
index e243ece..7e842a5 100644
--- a/androidmk/parser/make_strings_test.go
+++ b/androidmk/parser/make_strings_test.go
@@ -75,6 +75,16 @@
 			genMakeString(""),
 		},
 	},
+	{
+		// "x$(var1)y bar"
+		in:  genMakeString("x", "var1", "y bar"),
+		sep: " ",
+		n:   2,
+		expected: []*MakeString{
+			genMakeString("x", "var1", "y"),
+			genMakeString("bar"),
+		},
+	},
 }
 
 func TestMakeStringSplitN(t *testing.T) {