Fix panic in builds with no device

Running prebuilts/build-tools/build-prebuilts.sh in a repo that
contains art/build/art.go panics because config.Targets[Device] is
empty.  Check the length before accessing the slice.

Test: prebuilts/build-tools/build-prebuilts.sh
Change-Id: Ifb9fe0fad07b22d6b574f505c08c5c761278aad0
diff --git a/android/config.go b/android/config.go
index c3beb08..1f29d94 100644
--- a/android/config.go
+++ b/android/config.go
@@ -466,7 +466,11 @@
 }
 
 func (c *config) LibartImgDeviceBaseAddress() string {
-	switch c.Targets[Device][0].Arch.ArchType {
+	archType := Common
+	if len(c.Targets[Device]) > 0 {
+		archType = c.Targets[Device][0].Arch.ArchType
+	}
+	switch archType {
 	default:
 		return "0x70000000"
 	case Mips, Mips64: