Make aconfig flags generate a library instead of a srcjar.

Also add unit tests for the rest of device_config

Bug: 283475679
Test: m nothing (soong unit tests)
Change-Id: Iee18a1f2f2cbb23e8c8d84c54e903b32be29a693

java/builder.go

@@ -305,6 +305,12 @@ type javaBuilderFlags struct {
 	proto android.ProtoFlags
 }
 
+func DefaultJavaBuilderFlags() javaBuilderFlags {
+	return javaBuilderFlags{
+		javaVersion: JAVA_VERSION_8,
+	}
+}
+
 func TransformJavaToClasses(ctx android.ModuleContext, outputFile android.WritablePath, shardIdx int,
 	srcFiles, srcJars android.Paths, flags javaBuilderFlags, deps android.Paths) {