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

In order to implement our scheduling tuning policy in certain cases, we
need to initialize the variables that we have defined in the
user_struct. To achieve this, we will add a vendor hook to user.c at
alloc_uid, which will ensure that our own logic is executed during the
initialization of the user_struct.

Bug: 187458531

Signed-off-by: heshuai1 <heshuai1@xiaomi.com>
Change-Id: I078484aac2c3d396aba5971d6d0f491652f3781c
(cherry picked from commit c9b8fa64)
(cherry picked from commit 9ac0923e)