Handle boot jars on /system_ext correctly.

When Soong looks for boot jars among all modules, it applies certain
constraints to each module that looks like a boot jar (e.g. that it
comes from the right apex or platform). Previously these constraints did
not handle boot jars on /system_ext correctly (they were handled like
apex jars, while they should be handled like platform jars).

Bug: 154976937
Test: m nothing (the modified Soong test would fail previously)
Change-Id: I0746a2fd276ab5ef0400340c5b61cf26c2570e5a
diff --git a/java/dexpreopt_bootjars.go b/java/dexpreopt_bootjars.go
index 7137f33..e57c3e9 100644
--- a/java/dexpreopt_bootjars.go
+++ b/java/dexpreopt_bootjars.go
@@ -484,7 +484,7 @@
 
 	// Now match the apex part of the boot image configuration.
 	requiredApex := bootjars.Apex(index)
-	if requiredApex == "platform" {
+	if requiredApex == "platform" || requiredApex == "system_ext" {
 		if len(apexInfo.InApexes) != 0 {
 			// A platform variant is required but this is for an apex so ignore it.
 			return -1, nil, nil