Multi-user customizable quick affordances.

This CL adds support for multi-users to the customizable lock screen
quick affordance system.

The process that creates windows and renders System UI is always the
primary user's system UI process. The process that owns the content
provider accessed by Wallpaper Picker is bound to the currently-selected
user on the system, which may be different than the primary user.

What this means is that, when switching to a secondary user, what the
Wallpaper Picker sees is fed to it from a secondary system UI process
which is not the same system UI process that is rendering the
affordances on the lock screen. Therefore, it didn't work.

This CL adds a "remote user" selection manager which the repository can
switch to when it's running on the primary user process but needs to
query the state of the selected affordances for a secondary user.

The "remote user" selection manager queries the content provider
associated with the system UI process linked to the secondary user. This
way, we can display the correct affordances on the screen, even when
switching to a secondary user.

Fix: 260251307
Test: included new and expanded unit tests. Manually verified that the
selection of quick affordances for the primary and secondary user are
correctly reflected on the lock screen when switching users and is
retained even after switching away and back.

Change-Id: I281577ed6efb987c23b19c2078e77c91e45ce9f2