cmdline property in bootimg can be customized for debuggable builds

This change adds product_variables.debuggable.cmdline so that the kernel
cmdline arguments can be augmented for the debuggable builds.

To support that the type of the property has changed to string array.

Bug: 181936135
Test: inspect build.ninja
Change-Id: I81b3ead5187ccb378efabb87d1d15fbdb59d8e2f

android/variable.go

@@ -108,6 +108,8 @@ type variableProperties struct {
 			Static_libs       []string
 			Whole_static_libs []string
 			Shared_libs       []string
+
+			Cmdline []string
 		}
 
 		// eng is true for -eng builds, and can be used to turn on additionaly heavyweight debugging