Add product_variables.product_is_iot variables

These are enough to compile bootanimation with Soong.

Test: build bootanimation on internal branch
Change-Id: I8474b54ea196ab71ce178c517eb261bdc22b2e01

android/variable.go

@@ -65,6 +65,12 @@ type variableProperties struct {
 		// Product_is_iot is true for Android Things devices.
 		Product_is_iot struct {
 			Cflags       []string
+			Enabled      bool
+			Exclude_srcs []string
+			Init_rc      []string
+			Shared_libs  []string
+			Srcs         []string
+			Static_libs  []string
 		}
 
 		// treble_linker_namespaces is true when the system/vendor linker namespace separation is