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

tools/aconfig/templates/Flags.java.template

@@ -0,0 +1,11 @@
+package {package_name};
+
+public final class Flags \{
+    {{ for item in class_elements}}
+    public static boolean {item.method_name}() \{
+        return FEATURE_FLAGS.{item.method_name}();
+    }
+    {{ endfor }}
+    private static FeatureFlags FEATURE_FLAGS = new FeatureFlagsImpl();
+
+}