Add ProductVariables.DeviceName, set defaults to flounder

Using flounder defaults to compare against make.

Change-Id: I88fb0809e84b78ef005b76f255017a4d4a35fd13
diff --git a/common/config.go b/common/config.go
index 1cd75d3..1340db8 100644
--- a/common/config.go
+++ b/common/config.go
@@ -211,7 +211,7 @@
 // DeviceName returns the name of the current device target
 // TODO: take an AndroidModuleContext to select the device name for multi-device builds
 func (c *config) DeviceName() string {
-	return "unset"
+	return *c.ProductVariables.DeviceName
 }
 
 // DeviceOut returns the path to out directory for device targets