5/ Fix inverted dependency for ShellCommandHandler

- Instead of ShellCommandHandler referencing the controllers that it
  needs to dump/call on, invert it so that controllers register a
  dump/command callback when they are initialized

Test: atest WMShellUnitTests
Test: atest SystemUITests
Test: adb shell dumpsys activity service SystemUIservice WMShell
Test: adb shell dumpsys activity service SystemUIservice WMShell splitscreen <cmd>
Test: adb shell dumpsys activity service SystemUIservice WMShell help
Bug: 238217847
Change-Id: I673e5f7dd11ed7af326d52da8e793356169f1d8f