add PlatformAconfigPackageInternal

This commit adds PlatformAconfigPackageInternal. This class will be used
in generated flag code to read the flag value internally.

Test: atest aconfig_storage_file.test.java aconfig_storage_read_functional
aconfig_storage_read_unit
Bug: 367765164
Flag: NONE refactor
Ignore-AOSP-First: will cherry pick into aosp later
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:25cbc38585adc616986ccb5cd88b0b9268863dda)
Merged-In: Ie24e5d5585ff8a7d287ef62a4a58b58b697f81d6
Change-Id: Ie24e5d5585ff8a7d287ef62a4a58b58b697f81d6