Add pdk product variable

The pdk product variable will be set by make when TARGET_BUILD_PDK
is set, and can be used to disable modules that won't build in the
PDK.

Test: builds
Bug: 62086238
Change-Id: I2191a57b9b06680fd741308e7464275a89773530
diff --git a/android/variable.go b/android/variable.go
index 666d729..05f50b5 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -138,6 +138,7 @@
 	EnableCFI                  *bool `json:",omitempty"`
 	Device_uses_hwc2           *bool `json:",omitempty"`
 	Treble                     *bool `json:",omitempty"`
+	Pdk                        *bool `json:",omitempty"`
 
 	VendorPath *string `json:",omitempty"`