Let deps property be conditional on debuggability

This allows us to add dependencies only for debuggable builds.

Bug: N/A
Test: add product_variables.debuggable.deps to the android_system_image
module.

Change-Id: Id90646fff8a8a91c5e768418b9881e93228ec2a5

android/variable.go

@@ -139,6 +139,8 @@ type variableProperties struct {
 			Srcs         []string
 			Exclude_srcs []string
 			Cmd          *string
+
+			Deps []string
 		}
 
 		// eng is true for -eng builds, and can be used to turn on additional heavyweight debugging