Replace lists of aconfig modules with aconfig_declarations_group

Currently in AconfigFlags.bp, two list of aconfig modules are being
maintained: the list of java_aconfig_library modules and the list of
aconfig_declarations modules. When a new aconfig module is being added,
a developer must modify the two lists, and there is no guarantee that
the two lists are kept in sync. In order to resolve such issue, this
change replaces the two lists with the newly introduced
aconfig_declarations_group module.

Test: m
Bug: 320492079
Change-Id: Ib32c06de4f0279a17e5c65314e984265592d004c