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/src/commands.rs b/tools/aconfig/src/commands.rs
index 586ba04..eb860b0 100644
--- a/tools/aconfig/src/commands.rs
+++ b/tools/aconfig/src/commands.rs
@@ -89,7 +89,7 @@
     Ok(builder.build())
 }
 
-pub fn create_java_lib(cache: Cache) -> Result<OutputFile> {
+pub fn create_java_lib(cache: Cache) -> Result<Vec<OutputFile>> {
     generate_java_code(&cache)
 }