aconfig: Java codegen iteration 1

This change includes
    - refactor generated java code to generate
        - Flags.java to support the static API
	- FeatureFlagsImpl.java to support injection API
	- FeatureFlags.java interface

Bug: 279483816
Test: atest aconfig.test aconfig.test.java
Change-Id: If0d4baf317b9174635cd0fff3832ab7091ee52ed
diff --git a/tools/aconfig/templates/FeatureFlags.java.template b/tools/aconfig/templates/FeatureFlags.java.template
new file mode 100644
index 0000000..b9e2cc7
--- /dev/null
+++ b/tools/aconfig/templates/FeatureFlags.java.template
@@ -0,0 +1,7 @@
+package {package_name};
+
+public interface FeatureFlags \{
+    {{ for item in class_elements}}
+    boolean {item.method_name}();
+    {{ endfor }}
+}
\ No newline at end of file