aconfig: follow Java conventions for Java file paths

Update codegen_java to write the generated Java file(s) to
"java/package/File.java" instead of just "File.java".

Also generalize codegen_java::GeneratedFile to commands::OutputFile in
preparation for the upcoming C++ and Rust codegen.

Also change Java package name to 'com.android.internal.aconfig'.

Bug: 279485059
Test: atest aconfig.test
Change-Id: I13978697e35010fe6be8637aa495d4b852dbed7e