genrule: expand $$ to $$

An escaped $ sign in an expanded string needs to be re-escaped so it
can pass through to ninja.

Test: m -j libLLVMObject
Change-Id: I2f7890417fd6f534e6e21cace3d86ae9ef708ce5
diff --git a/android/expand_test.go b/android/expand_test.go
index ca50b39..128de8a 100644
--- a/android/expand_test.go
+++ b/android/expand_test.go
@@ -85,19 +85,19 @@
 	},
 	{
 		in:  "$$",
-		out: "$",
+		out: "$$",
 	},
 	{
 		in:  "$$(var1)",
-		out: "$(var1)",
+		out: "$$(var1)",
 	},
 	{
 		in:  "$$$(var1)",
-		out: "$abc",
+		out: "$$abc",
 	},
 	{
 		in:  "$(var1)$$",
-		out: "abc$",
+		out: "abc$$",
 	},
 	{
 		in:  "$(💩)",