Add new properties to aid in removing the 1-variant fallback

These new properties are essentially methods to specify "outgoing
transitions" in blueprint files. There are lots of host tests
that want to include apps built for device in their data, so they
need a property that adds dependencies based on the device variants
instead of copying the same host variants.

After this cl is submitted, I'll do an LSC to update all the usages
that are relying on the 1-variant fallback to use these properties
instead.

Bug: 372091092
Test: m nothing --no-skip-soong-tests
Change-Id: I45b8fb024da120ad61606e3a21de86e4392be2a4
diff --git a/filesystem/bootimg.go b/filesystem/bootimg.go
index e796ab9..9d93925 100644
--- a/filesystem/bootimg.go
+++ b/filesystem/bootimg.go
@@ -75,7 +75,7 @@
 
 	// Path to the private key that avbtool will use to sign this filesystem image.
 	// TODO(jiyong): allow apex_key to be specified here
-	Avb_private_key *string `android:"path"`
+	Avb_private_key *string `android:"path_device_first"`
 
 	// Hash and signing algorithm for avbtool. Default is SHA256_RSA4096.
 	Avb_algorithm *string