Pipe user changed events to the Shell

- Move PipController and OneHandedController to use this call internally
  instead of exposing it to SysUI

Bug: 238217847
Test: atest WMShellUnitTests
Test: atest PinnedStackTests
Test: Test switching users with PIP & One handed mode
Test: enable WM_SHELL_SYSUI_EVENTS protologs and verify
Change-Id: I0f016aabb945eb8ce80ee4d4203122659d668b8f