ANDROID: user: Add vendor hook to user for GKI purpose

Add the vendor hook to user.c, because of some speical cases related to
our feature, we need to initialize the variables defined by ourselves in
user_struct, so we add the hook at alloc_uid to make sure we can go to
our own logic when the user_struct is about to initialize.

Bug: 187458531

Signed-off-by: heshuai1 <heshuai1@xiaomi.com>
Change-Id: I078484aac2c3d396aba5971d6d0f491652f3781c