create AconfigPackageImpl

AconfigPackageImpl is the real implementation class for AconfigPackage.
Later AconfigPackage (for external), system AconfigPackageLocal, and
ConfigInfra AconfigPackageLocal will use this implemenation.

The benifits of extracting the implementation into this file are:

1. remove duplicated code
2. easy for testing

Test: atest aconfig_storage_package aconfig_storage_file.test.java
Bug: 367765164
Flag: EXEMPT refactor
Change-Id: Icce22b090f2a9483148b8cc60eed002e3d97f719