Add new aconfig flag to enable system contacts picker

The new flag will guard the ContactsPicker application components in the manifest file as well as the new ACTION_PICK_CONTACTS intent action.

Bug: 427934258
Flag: android.content.flags.enable_system_contacts_picker
Test: TH
Change-Id: I6bc031edcb4fa47452c1f0d524eb7d605252b65d