Make IMMS#mVisibilityStateComputer multi-user aware

This is a mechanical CL to instantiate ImeVisibilityStateComputer for
each user.

There must be no observable behavior change in the single user mode.

Fix: 349904272
Test: presubmit
Test: atest FrameworksInputMethodSystemServerTests
Flag: android.view.inputmethod.concurrent_input_methods
Change-Id: I296c8fbca87100178e30c0bb529fbcade377ec50