Fix vulnerability by explicitly set the class name of avatar picker.

Flag: android.multiuser.fix_avatar_cross_user_leak
Test: manual
Bug: 341688848
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4b731d5c8715190b88a44c443d5028b3014cd495)
Merged-In: I0203403d7724076c7d6266a7b5930fa26a3e54b7
Change-Id: I0203403d7724076c7d6266a7b5930fa26a3e54b7