aconfig:add system_ext in the platform list

system_ext is added back as a separate partition on the device. Missing
it from the list the generated code will use AconfigPackage instead of
the PlatforAconfigPackage to access the flag value.

Test: m com_android_launcher3_flags_lib and check the generated code
uses the right class
Bug: 402133645

Change-Id: Ibc0bce78ec4c540be12f32418d4d241e20aacbc5