Don't panic when DevicePrimaryArchType is called in a host only build

Check the length of c.Targets[Android] before getting the first
element, and return Common if the length is set.  Fixes a panic
when a tradefed_binary is defined in a host-only build.

Test: aosp-build-tools build
Change-Id: I89f1dd13aa1aff5eb7d39e0b66eeedcb7d930fa0
diff --git a/android/config.go b/android/config.go
index 8e01e18..4a4da08 100644
--- a/android/config.go
+++ b/android/config.go
@@ -892,8 +892,13 @@
 	return Bool(c.productVariables.Eng)
 }
 
+// DevicePrimaryArchType returns the ArchType for the first configured device architecture, or
+// Common if there are no device architectures.
 func (c *config) DevicePrimaryArchType() ArchType {
-	return c.Targets[Android][0].Arch.ArchType
+	if androidTargets := c.Targets[Android]; len(androidTargets) > 0 {
+		return androidTargets[0].Arch.ArchType
+	}
+	return Common
 }
 
 func (c *config) SanitizeHost() []string {