Add Product_is_iot to soong config.

Some code needs to behave differently for Android Things
systems. The Product_is_iot variable property can be used to
determine if a build is for Android Things or not.

Bug: 110494430
Test: aosp_angler builds
Change-Id: I902b438a334786adbd515188ad35e241d82e69fe

android/variable.go

@@ -62,6 +62,11 @@ type variableProperties struct {
 			Cflags []string
 		}
 
+		// Product_is_iot is true for Android Things devices.
+		Product_is_iot struct {
+			Cflags []string
+		}
+
 		// treble_linker_namespaces is true when the system/vendor linker namespace separation is
 		// enabled.
 		Treble_linker_namespaces struct {
@@ -201,6 +206,8 @@ type productVariables struct {
 
 	Override_rs_driver *string `json:",omitempty"`
 
+	Product_is_iot *bool `json:",omitempty"`
+
 	DeviceKernelHeaders []string `json:",omitempty"`
 	DistDir             *string  `json:",omitempty"`