Add a variable to disable modules in the PDK

Not plumbed through yet, but will at least prevent us from losing
track of which modules will not build in the PDK.

Test: builds
Change-Id: I4c1a0e39104b104dc1ff23c4391d5e83b3002d3a

android/variable.go

@@ -81,6 +81,10 @@ type variableProperties struct {
 			Cflags   []string
 			Cppflags []string
 		}
+
+		Pdk struct {
+			Enabled *bool
+		}
 	} `android:"arch_variant"`
 }
 

androidmk/cmd/androidmk/android.go

@@ -637,8 +637,11 @@ var conditionalTranslations = map[string]map[bool]string{
 		true:  "target.linux",
 		false: "target.not_linux"},
 	"(,$(TARGET_BUILD_APPS))": {
-		false: "product_variables.unbundled_build",
-	},
+		false: "product_variables.unbundled_build"},
+	"($(TARGET_BUILD_PDK),true)": {
+		true: "product_variables.pdk"},
+	"($(TARGET_BUILD_PDK), true)": {
+		true: "product_variables.pdk"},
 }
 
 func mydir(args []string) string {